diff --git a/app/(dashboard)/customers/[id]/page.tsx b/app/(dashboard)/customers/[id]/page.tsx
index 6496406f..7887f8a0 100644
--- a/app/(dashboard)/customers/[id]/page.tsx
+++ b/app/(dashboard)/customers/[id]/page.tsx
@@ -281,6 +281,12 @@ export default function CustomerDetailPage({
             <CardTitle className="text-base">{t('section_business')}</CardTitle>
           </CardHeader>
           <CardContent className="space-y-3">
+            {customer.customer_number && (
+              <div className="text-sm">
+                <span className="text-muted-foreground">{t('label_customer_number')} </span>
+                {customer.customer_number}
+              </div>
+            )}
             {customer.org_number && (
               <div className="text-sm">
                 <span className="text-muted-foreground">{t('label_org_number')} </span>
@@ -300,7 +306,7 @@ export default function CustomerDetailPage({
               <span className="text-muted-foreground">{t('label_payment_terms')} </span>
               {t('payment_terms_value', { days: customer.default_payment_terms || 30 })}
             </div>
-            {!customer.org_number && !customer.vat_number && (
+            {!customer.customer_number && !customer.org_number && !customer.vat_number && (
               <p className="text-sm text-muted-foreground">{t('no_business_info')}</p>
             )}
           </CardContent>
@@ -393,6 +399,7 @@ export default function CustomerDetailPage({
             initialData={{
               name: customer.name,
               customer_type: customer.customer_type,
+              customer_number: customer.customer_number || undefined,
               email: customer.email || undefined,
               phone: customer.phone || undefined,
               address_line1: customer.address_line1 || undefined,
diff --git a/app/api/customers/[id]/route.ts b/app/api/customers/[id]/route.ts
index e22b7f6a..0964ef67 100644
--- a/app/api/customers/[id]/route.ts
+++ b/app/api/customers/[id]/route.ts
@@ -58,6 +58,8 @@ export const PATCH = withRouteContext(
     const updateData: Record<string, unknown> = {}
     if (body.name !== undefined) updateData.name = body.name
     if (body.customer_type !== undefined) updateData.customer_type = body.customer_type
+    // Empty string clears the customer number, same as an explicit null.
+    if (body.customer_number !== undefined) updateData.customer_number = body.customer_number || null
     if (body.email !== undefined) updateData.email = body.email
     if (body.phone !== undefined) updateData.phone = body.phone
     if (body.address_line1 !== undefined) updateData.address_line1 = body.address_line1
diff --git a/app/api/customers/__tests__/customer-number.test.ts b/app/api/customers/__tests__/customer-number.test.ts
new file mode 100644
index 00000000..583104b9
--- /dev/null
+++ b/app/api/customers/__tests__/customer-number.test.ts
@@ -0,0 +1,230 @@
+/**
+ * Tests for the customer_number field (kundnummer, issue #914) on
+ * POST /api/customers and PATCH /api/customers/[id].
+ *
+ * Exercises the routes through the real withRouteContext wrapper, mocking its
+ * auth/company/write dependencies. Uses a hand-rolled Supabase mock that
+ * records insert/update payloads so the tests can assert the route-level
+ * normalization: the value is trimmed by the Zod schema, and empty string or
+ * null clears the column to null.
+ */
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { createMockRequest, parseJsonResponse } from '@/tests/helpers'
+import { eventBus } from '@/lib/events'
+
+const captured: { insert: unknown[]; update: unknown[] } = { insert: [], update: [] }
+let queryResult: { data: unknown; error: unknown } = { data: null, error: null }
+
+const buildChain = (): unknown =>
+  new Proxy(
+    {},
+    {
+      get(_target, prop) {
+        if (prop === 'then') {
+          return (resolve: (v: unknown) => void) => resolve(queryResult)
+        }
+        return (...args: unknown[]) => {
+          if (prop === 'insert') captured.insert.push(args[0])
+          if (prop === 'update') captured.update.push(args[0])
+          return buildChain()
+        }
+      },
+    },
+  )
+
+const supabase = {
+  from: vi.fn(() => buildChain()),
+  rpc: vi.fn(() => buildChain()),
+}
+
+const requireAuthMock = vi.fn()
+vi.mock('@/lib/auth/require-auth', () => ({
+  requireAuth: (...args: unknown[]) => requireAuthMock(...args),
+}))
+
+vi.mock('@/lib/company/context', () => ({
+  getActiveCompanyId: vi.fn().mockResolvedValue('company-1'),
+  requireCompanyId: vi.fn().mockResolvedValue('company-1'),
+}))
+
+const requireWriteMock = vi.fn()
+vi.mock('@/lib/auth/require-write', () => ({
+  requireWritePermission: (...args: unknown[]) => requireWriteMock(...args),
+}))
+
+vi.mock('@/lib/init', () => ({ ensureInitialized: vi.fn() }))
+
+import { POST } from '../route'
+import { PATCH } from '../[id]/route'
+
+type CustomerRow = { customer_number?: string | null }
+
+describe('customer_number on POST /api/customers', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    eventBus.clear()
+    captured.insert.length = 0
+    captured.update.length = 0
+    queryResult = { data: null, error: null }
+    requireAuthMock.mockResolvedValue({ user: { id: 'user-1' }, supabase })
+    requireWriteMock.mockResolvedValue({ ok: true })
+  })
+
+  it('rejects a customer_number longer than 32 characters with 400', async () => {
+    const request = createMockRequest('/api/customers', {
+      method: 'POST',
+      body: {
+        name: 'Test AB',
+        customer_type: 'swedish_business',
+        customer_number: 'X'.repeat(33),
+      },
+    })
+
+    const response = await POST(request, { params: Promise.resolve({}) })
+    const { status } = await parseJsonResponse(response)
+
+    expect(status).toBe(400)
+    expect(captured.insert).toHaveLength(0)
+  })
+
+  it('stores a trimmed customer_number on create', async () => {
+    queryResult = {
+      data: { id: 'cust-1', name: 'Test AB', customer_type: 'swedish_business', customer_number: '1001' },
+      error: null,
+    }
+
+    const request = createMockRequest('/api/customers', {
+      method: 'POST',
+      body: {
+        name: 'Test AB',
+        customer_type: 'swedish_business',
+        customer_number: '  1001  ',
+      },
+    })
+
+    const response = await POST(request, { params: Promise.resolve({}) })
+    const { status, body } = await parseJsonResponse<{ data: CustomerRow }>(response)
+
+    expect(status).toBe(200)
+    expect((captured.insert[0] as CustomerRow).customer_number).toBe('1001')
+    expect(body.data.customer_number).toBe('1001')
+  })
+
+  it('normalizes an empty customer_number to null on create', async () => {
+    queryResult = {
+      data: { id: 'cust-1', name: 'Test AB', customer_type: 'swedish_business', customer_number: null },
+      error: null,
+    }
+
+    const request = createMockRequest('/api/customers', {
+      method: 'POST',
+      body: { name: 'Test AB', customer_type: 'swedish_business', customer_number: '' },
+    })
+
+    const response = await POST(request, { params: Promise.resolve({}) })
+    const { status } = await parseJsonResponse(response)
+
+    expect(status).toBe(200)
+    expect((captured.insert[0] as CustomerRow).customer_number).toBeNull()
+  })
+
+  it('defaults customer_number to null when omitted', async () => {
+    queryResult = {
+      data: { id: 'cust-1', name: 'Test AB', customer_type: 'swedish_business', customer_number: null },
+      error: null,
+    }
+
+    const request = createMockRequest('/api/customers', {
+      method: 'POST',
+      body: { name: 'Test AB', customer_type: 'swedish_business' },
+    })
+
+    const response = await POST(request, { params: Promise.resolve({}) })
+    const { status } = await parseJsonResponse(response)
+
+    expect(status).toBe(200)
+    expect((captured.insert[0] as CustomerRow).customer_number).toBeNull()
+  })
+})
+
+describe('customer_number on PATCH /api/customers/[id]', () => {
+  const routeParams = { params: Promise.resolve({ id: 'cust-1' }) }
+
+  beforeEach(() => {
+    vi.clearAllMocks()
+    eventBus.clear()
+    captured.insert.length = 0
+    captured.update.length = 0
+    queryResult = {
+      data: { id: 'cust-1', customer_type: 'swedish_business' },
+      error: null,
+    }
+    requireAuthMock.mockResolvedValue({ user: { id: 'user-1' }, supabase })
+    requireWriteMock.mockResolvedValue({ ok: true })
+  })
+
+  it('rejects a customer_number longer than 32 characters with 400', async () => {
+    const request = createMockRequest('/api/customers/cust-1', {
+      method: 'PATCH',
+      body: { customer_number: 'X'.repeat(33) },
+    })
+
+    const response = await PATCH(request, routeParams)
+    const { status } = await parseJsonResponse(response)
+
+    expect(status).toBe(400)
+    expect(captured.update).toHaveLength(0)
+  })
+
+  it('updates the customer_number', async () => {
+    const request = createMockRequest('/api/customers/cust-1', {
+      method: 'PATCH',
+      body: { customer_number: 'K-42' },
+    })
+
+    const response = await PATCH(request, routeParams)
+    const { status } = await parseJsonResponse(response)
+
+    expect(status).toBe(200)
+    expect((captured.update[0] as CustomerRow).customer_number).toBe('K-42')
+  })
+
+  it('clears the customer_number when null is sent', async () => {
+    const request = createMockRequest('/api/customers/cust-1', {
+      method: 'PATCH',
+      body: { customer_number: null },
+    })
+
+    const response = await PATCH(request, routeParams)
+    const { status } = await parseJsonResponse(response)
+
+    expect(status).toBe(200)
+    expect((captured.update[0] as CustomerRow).customer_number).toBeNull()
+  })
+
+  it('clears the customer_number when an empty string is sent', async () => {
+    const request = createMockRequest('/api/customers/cust-1', {
+      method: 'PATCH',
+      body: { customer_number: '' },
+    })
+
+    const response = await PATCH(request, routeParams)
+    const { status } = await parseJsonResponse(response)
+
+    expect(status).toBe(200)
+    expect((captured.update[0] as CustomerRow).customer_number).toBeNull()
+  })
+
+  it('leaves the customer_number untouched when the field is omitted', async () => {
+    const request = createMockRequest('/api/customers/cust-1', {
+      method: 'PATCH',
+      body: { name: 'New Name AB' },
+    })
+
+    const response = await PATCH(request, routeParams)
+    const { status } = await parseJsonResponse(response)
+
+    expect(status).toBe(200)
+    expect(captured.update[0]).not.toHaveProperty('customer_number')
+  })
+})
diff --git a/app/api/customers/route.ts b/app/api/customers/route.ts
index 4342c8ad..10bb84c3 100644
--- a/app/api/customers/route.ts
+++ b/app/api/customers/route.ts
@@ -49,6 +49,7 @@ export const POST = withRouteContext(
         company_id: companyId,
         name: body.name,
         customer_type: body.customer_type,
+        customer_number: body.customer_number || null,
         email: body.email,
         phone: body.phone,
         address_line1: body.address_line1,
diff --git a/app/api/invoices/preview-pdf/route.ts b/app/api/invoices/preview-pdf/route.ts
index 522c3580..068da7ea 100644
--- a/app/api/invoices/preview-pdf/route.ts
+++ b/app/api/invoices/preview-pdf/route.ts
@@ -44,6 +44,7 @@ export const POST = withRouteContext('invoice.preview_pdf', async (request, { su
       company_id: 'preview-company',
       name: 'Exempel AB',
       customer_type: 'swedish_business',
+      customer_number: null,
       email: 'kund@exempel.se',
       phone: null,
       address_line1: 'Storgatan 1',
diff --git a/app/api/v1/companies/[companyId]/customers/[id]/route.ts b/app/api/v1/companies/[companyId]/customers/[id]/route.ts
index 80e38154..994ea3bb 100644
--- a/app/api/v1/companies/[companyId]/customers/[id]/route.ts
+++ b/app/api/v1/companies/[companyId]/customers/[id]/route.ts
@@ -33,6 +33,7 @@ const CustomerDetail = z.object({
   id: z.string().uuid(),
   name: z.string(),
   customer_type: z.string(),
+  customer_number: z.string().nullable(),
   email: z.string().nullable(),
   phone: z.string().nullable(),
   address_line1: z.string().nullable(),
@@ -56,7 +57,7 @@ const OPEN_INVOICE_STATUSES = ['sent', 'partially_paid', 'overdue']
 // Explicit projection. Excludes user_id, company_id (internal scoping),
 // and vat_number_validated_at (internal timestamp not in the public schema).
 const CUSTOMER_DETAIL_COLUMNS =
-  'id, name, customer_type, email, phone, address_line1, address_line2, postal_code, city, country, org_number, vat_number, vat_number_validated, default_payment_terms, notes, archived_at, created_at, updated_at'
+  'id, name, customer_type, customer_number, email, phone, address_line1, address_line2, postal_code, city, country, org_number, vat_number, vat_number_validated, default_payment_terms, notes, archived_at, created_at, updated_at'
 
 const OPEN_INVOICE_COLUMNS =
   'id, invoice_number, invoice_date, due_date, status, currency, total, remaining_amount'
@@ -310,6 +311,11 @@ export const PATCH = withApiV1<{ params: Promise<{ companyId: string; id: string
     ] as const) {
       if (body[key] !== undefined) updateData[key] = body[key]
     }
+    // Empty string clears the customer number, same as an explicit null
+    // (matches the internal /api/customers route).
+    if (body.customer_number !== undefined) {
+      updateData.customer_number = body.customer_number || null
+    }
 
     if (Object.keys(updateData).length === 0) {
       return v1ErrorResponseFromCode('VALIDATION_ERROR', ctx.log, {
diff --git a/app/api/v1/companies/[companyId]/customers/__tests__/route.test.ts b/app/api/v1/companies/[companyId]/customers/__tests__/route.test.ts
index 7431023a..b8fa73a6 100644
--- a/app/api/v1/companies/[companyId]/customers/__tests__/route.test.ts
+++ b/app/api/v1/companies/[companyId]/customers/__tests__/route.test.ts
@@ -51,6 +51,13 @@ const mockValidate = validateApiKey as ReturnType<typeof vi.fn>
 const mockServiceClient = createServiceClientNoCookies as ReturnType<typeof vi.fn>
 
 function makeFlexibleSupabase(byTable: Record<string, { data?: unknown; error?: unknown }>) {
+  // Records insert/update payloads and .select() projection strings so
+  // tests can assert what the route writes and which columns it fetches.
+  const captured: {
+    insert: unknown[]
+    update: unknown[]
+    selects: Record<string, string[]>
+  } = { insert: [], update: [], selects: {} }
   const buildChain = (table: string): unknown => {
     const handler: ProxyHandler<object> = {
       get(_target, prop) {
@@ -58,12 +65,19 @@ function makeFlexibleSupabase(byTable: Record<string, { data?: unknown; error?:
           return (resolve: (v: unknown) => void) =>
             resolve(byTable[table] ?? { data: null, error: null })
         }
-        return (..._args: unknown[]) => buildChain(table)
+        return (...args: unknown[]) => {
+          if (prop === 'insert') captured.insert.push(args[0])
+          if (prop === 'update') captured.update.push(args[0])
+          if (prop === 'select' && typeof args[0] === 'string') {
+            ;(captured.selects[table] ??= []).push(args[0])
+          }
+          return buildChain(table)
+        }
       },
     }
     return new Proxy({}, handler)
   }
-  return { from: vi.fn((table: string) => buildChain(table)) }
+  return { from: vi.fn((table: string) => buildChain(table)), captured }
 }
 
 const COMPANY_ID = 'aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa'
@@ -589,6 +603,55 @@ describe('POST /api/v1/companies/:companyId/customers', () => {
     const body = await res.json()
     expect(body.error.code).toBe('INSUFFICIENT_SCOPE')
   })
+
+  it('round-trips customer_number: persisted in the insert and selected back in the response', async () => {
+    withWriteScope()
+    const supabaseMock = makeFlexibleSupabase({
+      company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+      customers: { data: { ...SAMPLE_CUSTOMER, customer_number: '1001' }, error: null },
+    })
+    mockServiceClient.mockReturnValue(supabaseMock)
+
+    const res = await createCustomer(
+      makePostRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/customers`, {
+        name: 'Acme AB',
+        customer_type: 'swedish_business',
+        customer_number: '1001',
+      }),
+      companyParams(COMPANY_ID),
+    )
+
+    expect(res.status).toBe(201)
+    const body = await res.json()
+    expect(body.data.customer_number).toBe('1001')
+    // The insert payload carries the field (not silently dropped) …
+    const insertPayload = supabaseMock.captured.insert[0] as { customer_number?: string | null }
+    expect(insertPayload.customer_number).toBe('1001')
+    // … and the response projection selects it back.
+    expect(supabaseMock.captured.selects['customers']?.[0]).toContain('customer_number')
+  })
+
+  it('normalizes an empty customer_number to null on create', async () => {
+    withWriteScope()
+    const supabaseMock = makeFlexibleSupabase({
+      company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+      customers: { data: { ...SAMPLE_CUSTOMER, customer_number: null }, error: null },
+    })
+    mockServiceClient.mockReturnValue(supabaseMock)
+
+    const res = await createCustomer(
+      makePostRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/customers`, {
+        name: 'Acme AB',
+        customer_type: 'swedish_business',
+        customer_number: '',
+      }),
+      companyParams(COMPANY_ID),
+    )
+
+    expect(res.status).toBe(201)
+    const insertPayload = supabaseMock.captured.insert[0] as { customer_number?: string | null }
+    expect(insertPayload.customer_number).toBeNull()
+  })
 })
 
 // ──────────────────────────────────────────────────────────────────
@@ -722,6 +785,49 @@ describe('PATCH /api/v1/companies/:companyId/customers/:id', () => {
     expect(body.data.archived_at).toBeNull()
   })
 
+  it('round-trips customer_number: persisted in the update and selected back in the response', async () => {
+    withWriteScope()
+    const supabaseMock = makeFlexibleSupabase({
+      company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+      customers: { data: { ...SAMPLE_CUSTOMER, customer_number: 'K-42' }, error: null },
+    })
+    mockServiceClient.mockReturnValue(supabaseMock)
+
+    const res = await updateCustomer(
+      makePatchRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/customers/${CUSTOMER_ID}`, {
+        customer_number: 'K-42',
+      }),
+      detailParams(COMPANY_ID, CUSTOMER_ID),
+    )
+
+    expect(res.status).toBe(200)
+    const body = await res.json()
+    expect(body.data.customer_number).toBe('K-42')
+    const updatePayload = supabaseMock.captured.update[0] as { customer_number?: string | null }
+    expect(updatePayload.customer_number).toBe('K-42')
+    expect(supabaseMock.captured.selects['customers']?.[0]).toContain('customer_number')
+  })
+
+  it('clears customer_number when an empty string is sent', async () => {
+    withWriteScope()
+    const supabaseMock = makeFlexibleSupabase({
+      company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+      customers: { data: { ...SAMPLE_CUSTOMER, customer_number: null }, error: null },
+    })
+    mockServiceClient.mockReturnValue(supabaseMock)
+
+    const res = await updateCustomer(
+      makePatchRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/customers/${CUSTOMER_ID}`, {
+        customer_number: '',
+      }),
+      detailParams(COMPANY_ID, CUSTOMER_ID),
+    )
+
+    expect(res.status).toBe(200)
+    const updatePayload = supabaseMock.captured.update[0] as { customer_number?: string | null }
+    expect(updatePayload.customer_number).toBeNull()
+  })
+
   it('rejects an archived_at value that is not null', async () => {
     withWriteScope()
     mockServiceClient.mockReturnValue(
diff --git a/app/api/v1/companies/[companyId]/customers/route.ts b/app/api/v1/companies/[companyId]/customers/route.ts
index a887793c..41e2b557 100644
--- a/app/api/v1/companies/[companyId]/customers/route.ts
+++ b/app/api/v1/companies/[companyId]/customers/route.ts
@@ -232,6 +232,7 @@ const CustomerCreated = z.object({
   id: z.string().uuid().nullable(),
   name: z.string(),
   customer_type: CustomerType,
+  customer_number: z.string().nullable(),
   email: z.string().nullable(),
   phone: z.string().nullable(),
   address_line1: z.string().nullable(),
@@ -252,7 +253,7 @@ const CustomerCreated = z.object({
 // Drop vat_number_validated_at: declared in neither CustomerCreated nor
 // CustomerDetail; an internal timestamp with no documented consumer.
 const CUSTOMER_RESPONSE_COLUMNS =
-  'id, name, customer_type, email, phone, address_line1, address_line2, postal_code, city, country, org_number, vat_number, vat_number_validated, default_payment_terms, notes, archived_at, created_at, updated_at'
+  'id, name, customer_type, customer_number, email, phone, address_line1, address_line2, postal_code, city, country, org_number, vat_number, vat_number_validated, default_payment_terms, notes, archived_at, created_at, updated_at'
 
 registerEndpoint({
   operation: 'customers.create',
@@ -339,6 +340,7 @@ export const POST = withApiV1<{ params: Promise<{ companyId: string }> }>(
           id: null,
           name: body.name,
           customer_type: body.customer_type,
+          customer_number: body.customer_number || null,
           email: body.email ?? null,
           phone: body.phone ?? null,
           address_line1: body.address_line1 ?? null,
@@ -383,6 +385,9 @@ export const POST = withApiV1<{ params: Promise<{ companyId: string }> }>(
         company_id: ctx.companyId!,
         name: body.name,
         customer_type: body.customer_type,
+        // Empty string clears the customer number, same as an explicit null
+        // (matches the internal /api/customers route).
+        customer_number: body.customer_number || null,
         email: body.email ?? null,
         phone: body.phone ?? null,
         address_line1: body.address_line1 ?? null,
diff --git a/app/api/v1/companies/[companyId]/invoices/[id]/send/__tests__/route.test.ts b/app/api/v1/companies/[companyId]/invoices/[id]/send/__tests__/route.test.ts
index fe03a55e..130855dc 100644
--- a/app/api/v1/companies/[companyId]/invoices/[id]/send/__tests__/route.test.ts
+++ b/app/api/v1/companies/[companyId]/invoices/[id]/send/__tests__/route.test.ts
@@ -101,6 +101,9 @@ function makeFlexibleSupabase(byTable: Record<string, MockResult | MockResult[]>
   for (const [t, val] of Object.entries(byTable)) {
     queues.set(t, Array.isArray(val) ? [...val] : [val])
   }
+  // Records every .select() projection string per table so tests can assert
+  // which columns the route actually fetches.
+  const selects: Record<string, string[]> = {}
   const buildChain = (table: string): unknown => {
     const handler: ProxyHandler<object> = {
       get(_target, prop) {
@@ -111,12 +114,17 @@ function makeFlexibleSupabase(byTable: Record<string, MockResult | MockResult[]>
             resolve(next)
           }
         }
-        return (..._args: unknown[]) => buildChain(table)
+        return (...args: unknown[]) => {
+          if (prop === 'select' && typeof args[0] === 'string') {
+            ;(selects[table] ??= []).push(args[0])
+          }
+          return buildChain(table)
+        }
       },
     }
     return new Proxy({}, handler)
   }
-  return { from: vi.fn((table: string) => buildChain(table)) }
+  return { from: vi.fn((table: string) => buildChain(table)), selects }
 }
 
 const COMPANY_ID = 'aaaaaaaa-aaaa-4aaa-8aaa-aaaaaaaaaaaa'
@@ -416,6 +424,30 @@ describe('POST /api/v1/companies/:companyId/invoices/:id/send', () => {
     expect(finalRenderArgs.invoice.invoice_number).toBe('2026-0043')
   })
 
+  it('fetches customer_number in the customer join so the emailed PDF matches the downloaded one', async () => {
+    // The pdf route selects customers(*); this route uses an explicit
+    // projection. If customer_number is dropped here, the emailed PDF
+    // silently omits the kundnummer that the downloaded PDF shows.
+    const supabaseMock = makeFlexibleSupabase({
+      company_members: { data: { company_id: COMPANY_ID, role: 'owner' }, error: null },
+      invoices: [
+        { data: DRAFT_INVOICE, error: null },
+        { data: { invoice_number: '2026-0042' }, error: null },
+      ],
+      company_settings: { data: COMPANY_SETTINGS, error: null },
+    })
+    mockServiceClient.mockReturnValue(supabaseMock)
+
+    const res = await sendInvoice(
+      makeRequest(`https://x.test/api/v1/companies/${COMPANY_ID}/invoices/${INVOICE_ID}/send`),
+      detailParams(COMPANY_ID, INVOICE_ID),
+    )
+    expect(res.status).toBe(200)
+
+    const invoiceFetchProjection = supabaseMock.selects['invoices']?.[0] ?? ''
+    expect(invoiceFetchProjection).toMatch(/customer:customers\([^)]*\bcustomer_number\b[^)]*\)/)
+  })
+
   it('test-mode key forces dry-run: returns a preview, no email, no number burned', async () => {
     // A test key has no ?dry_run flag, but the wrapper forces dry-run because
     // the key is mode='test'. The send endpoint declares dryRunSupported, so the
diff --git a/app/api/v1/companies/[companyId]/invoices/[id]/send/route.ts b/app/api/v1/companies/[companyId]/invoices/[id]/send/route.ts
index 3bf8748c..e774d421 100644
--- a/app/api/v1/companies/[companyId]/invoices/[id]/send/route.ts
+++ b/app/api/v1/companies/[companyId]/invoices/[id]/send/route.ts
@@ -162,7 +162,7 @@ export const POST = withApiV1<{ params: Promise<{ companyId: string; id: string
     const { data: invoice, error: fetchErr } = await ctx.supabase
       .from('invoices')
       .select(
-        `${INVOICE_SEND_RESPONSE_COLUMNS}, customer:customers(id, name, email, customer_type, country, address_line1, address_line2, postal_code, city, vat_number), items:invoice_items(id, sort_order, description, quantity, unit, unit_price, line_total, vat_rate, vat_amount, revenue_account, dimensions)`,
+        `${INVOICE_SEND_RESPONSE_COLUMNS}, customer:customers(id, name, customer_number, email, customer_type, country, address_line1, address_line2, postal_code, city, vat_number), items:invoice_items(id, sort_order, description, quantity, unit, unit_price, line_total, vat_rate, vat_amount, revenue_account, dimensions)`,
       )
       .eq('company_id', ctx.companyId!)
       .eq('id', invoiceId)
diff --git a/components/customers/CustomerForm.tsx b/components/customers/CustomerForm.tsx
index 6ebfdad9..f865ecfb 100644
--- a/components/customers/CustomerForm.tsx
+++ b/components/customers/CustomerForm.tsx
@@ -38,6 +38,7 @@ export default function CustomerForm({
   const schema = useMemo(() => z.object({
     name: z.string().min(1, t('name_required')),
     customer_type: z.enum(['individual', 'swedish_business', 'eu_business', 'non_eu_business']),
+    customer_number: z.string().trim().max(32, t('customer_number_too_long')).optional(),
     email: z.string().email(t('email_invalid')).optional().or(z.literal('')),
     phone: z.string().optional(),
     address_line1: z.string().optional(),
@@ -70,6 +71,7 @@ export default function CustomerForm({
     defaultValues: {
       name: initialData?.name || '',
       customer_type: initialData?.customer_type || 'swedish_business',
+      customer_number: initialData?.customer_number || '',
       email: initialData?.email || '',
       phone: initialData?.phone || '',
       address_line1: initialData?.address_line1 || '',
@@ -178,6 +180,21 @@ export default function CustomerForm({
         )}
       </div>
 
+      {/* Customer number */}
+      <div className="space-y-2">
+        <Label htmlFor="customer_number">{t('customer_number_label')}</Label>
+        <Input
+          id="customer_number"
+          placeholder={t('customer_number_placeholder')}
+          {...register('customer_number')}
+        />
+        {errors.customer_number ? (
+          <p className="text-sm text-destructive">{errors.customer_number.message}</p>
+        ) : (
+          <p className="text-xs text-muted-foreground">{t('customer_number_hint')}</p>
+        )}
+      </div>
+
       {/* Contact */}
       <div className="grid grid-cols-1 sm:grid-cols-2 gap-4">
         <div className="space-y-2">
diff --git a/lib/api/schemas.ts b/lib/api/schemas.ts
index b6e587ca..97980ec1 100644
--- a/lib/api/schemas.ts
+++ b/lib/api/schemas.ts
@@ -601,6 +601,14 @@ export const MarkInvoicePaidSchema = z.object({
 export const CreateCustomerSchema = z.object({
   name: z.string().min(1, 'Customer name is required'),
   customer_type: CustomerTypeSchema,
+  // Kundnummer shown on invoices. Free text, not unique in v1. Empty string
+  // and null both clear the value (routes normalize '' to null).
+  customer_number: z
+    .string()
+    .trim()
+    .max(32, 'Customer number must be 32 characters or fewer')
+    .nullable()
+    .optional(),
   email: z.string().email('Invalid email address').optional(),
   phone: z.string().optional(),
   address_line1: z.string().optional(),
diff --git a/lib/invoices/pdf-template.tsx b/lib/invoices/pdf-template.tsx
index c1cdc0e5..acd4e2b0 100644
--- a/lib/invoices/pdf-template.tsx
+++ b/lib/invoices/pdf-template.tsx
@@ -42,6 +42,7 @@ const LABELS = {
     yourReference: 'Er referens:',
     ourReference: 'Vår referens:',
     // Customer box
+    custNo: 'Kundnr:',
     orgNo: 'Org.nr:',
     vat: 'VAT:',
     // Table columns
@@ -111,6 +112,7 @@ const LABELS = {
     deliveryDate: 'Delivery date:',
     yourReference: 'Your reference:',
     ourReference: 'Our reference:',
+    custNo: 'Customer no.:',
     orgNo: 'Reg. no.:',
     vat: 'VAT:',
     colDescription: 'Description',
@@ -799,6 +801,12 @@ export function InvoicePDF({ invoice, customer, items, company, originalInvoiceN
               {customer.country && customer.country !== 'SE' && (
                 <Text>{customer.country}</Text>
               )}
+              {/* Seller-assigned kundnummer: no per-customer-type guard needed,
+                  it identifies the customer in the seller's own register and
+                  carries no personal data of its own. */}
+              {customer.customer_number && (
+                <Text style={{ marginTop: 6 }}>{L.custNo} {customer.customer_number}</Text>
+              )}
               {/* Suppress the identifier row for private customers: their
                   personnummer is not required on a B2C invoice (ML 17 kap 24§
                   asks for name + address only) and printing it is a GDPR
diff --git a/messages/en.json b/messages/en.json
index 3d9b9505..1d81ed3b 100644
--- a/messages/en.json
+++ b/messages/en.json
@@ -660,6 +660,7 @@
     "no_contact_info": "No contact details",
     "no_business_info": "No business details",
     "no_invoices": "No invoices linked to this customer",
+    "label_customer_number": "Customer number:",
     "label_org_number": "Org. no.:",
     "label_vat": "VAT:",
     "label_payment_terms": "Payment terms:",
@@ -695,6 +696,10 @@
     "name_label": "Name *",
     "name_placeholder": "Company or person name",
     "name_required": "Name is required",
+    "customer_number_label": "Customer number",
+    "customer_number_placeholder": "E.g. 1001",
+    "customer_number_hint": "Shown on the invoice. Leave empty if you do not use customer numbers.",
+    "customer_number_too_long": "Customer number must be 32 characters or fewer",
     "email_label": "Email",
     "email_placeholder": "name@company.com",
     "email_invalid": "Invalid email address",
diff --git a/messages/sv.json b/messages/sv.json
index cac13a5e..db4f5fc7 100644
--- a/messages/sv.json
+++ b/messages/sv.json
@@ -660,6 +660,7 @@
     "no_contact_info": "Inga kontaktuppgifter",
     "no_business_info": "Inga företagsuppgifter",
     "no_invoices": "Inga fakturor kopplade till denna kund",
+    "label_customer_number": "Kundnummer:",
     "label_org_number": "Org.nr:",
     "label_vat": "VAT:",
     "label_payment_terms": "Betalningsvillkor:",
@@ -695,6 +696,10 @@
     "name_label": "Namn *",
     "name_placeholder": "Företagsnamn eller personnamn",
     "name_required": "Namn krävs",
+    "customer_number_label": "Kundnummer",
+    "customer_number_placeholder": "T.ex. 1001",
+    "customer_number_hint": "Visas på fakturan. Lämna tomt om du inte använder kundnummer.",
+    "customer_number_too_long": "Kundnumret får vara högst 32 tecken",
     "email_label": "E-post",
     "email_placeholder": "namn@foretag.se",
     "email_invalid": "Ogiltig e-postadress",
diff --git a/supabase/migrations/20260709140000_add_customer_number_to_customers.sql b/supabase/migrations/20260709140000_add_customer_number_to_customers.sql
new file mode 100644
index 00000000..02f6036a
--- /dev/null
+++ b/supabase/migrations/20260709140000_add_customer_number_to_customers.sql
@@ -0,0 +1,16 @@
+-- Customer number (kundnummer) on customers, shown on the invoice (issue #914).
+--
+-- Nullable free text set by the user. Deliberately NO unique constraint in v1:
+-- existing rows, register imports, and provider syncs must not start failing
+-- on duplicates. Auto-numbering and uniqueness can be layered on later.
+--
+-- No RLS changes needed: customers already has company-scoped policies and a
+-- plain column add inherits them.
+
+ALTER TABLE public.customers
+  ADD COLUMN IF NOT EXISTS customer_number text;
+
+COMMENT ON COLUMN public.customers.customer_number IS
+  'User-assigned customer number (kundnummer) printed on invoices. Free text, not unique in v1.';
+
+NOTIFY pgrst, 'reload schema';
diff --git a/tests/helpers.ts b/tests/helpers.ts
index 9035a118..cf2a6022 100644
--- a/tests/helpers.ts
+++ b/tests/helpers.ts
@@ -400,6 +400,7 @@ export function makeCustomer(overrides: Partial<Customer> = {}): Customer {
     company_id: 'company-1',
     name: 'Test AB',
     customer_type: 'swedish_business',
+    customer_number: null,
     email: 'kontakt@test.se',
     phone: null,
     address_line1: 'Storgatan 1',
diff --git a/types/index.ts b/types/index.ts
index 3bc590f9..088229e0 100644
--- a/types/index.ts
+++ b/types/index.ts
@@ -557,6 +557,10 @@ export interface Customer {
   name: string
   customer_type: CustomerType
 
+  // User-assigned customer number (kundnummer) shown on invoices.
+  // Free text, no uniqueness enforced in v1.
+  customer_number: string | null
+
   // Contact
   email: string | null
   phone: string | null
@@ -1129,6 +1133,7 @@ export interface TaxRate {
 export interface CreateCustomerInput {
   name: string
   customer_type: CustomerType
+  customer_number?: string | null
   email?: string
   phone?: string
   address_line1?: string