diff --git a/supabase/migrations/20260418150000_company_settings_default_voucher_series.sql b/supabase/migrations/20260418150000_company_settings_default_voucher_series.sql
new file mode 100644
index 00000000..e0fb1175
--- /dev/null
+++ b/supabase/migrations/20260418150000_company_settings_default_voucher_series.sql
@@ -0,0 +1,41 @@
+-- Add default_voucher_series to company_settings.
+--
+-- The frontend (/settings/bookkeeping) and TypeScript types have referenced
+-- this column for a while, but no migration ever added it, so saving the form
+-- failed with: "Could not find the 'default_voucher_series' column of
+-- 'company_settings' in the schema cache".
+--
+-- The column stores the per-company default voucher series (A–Z) that is
+-- pre-selected when booking manual journal entries. The actual sequence
+-- counters live in public.voucher_sequences, keyed on (user_id,
+-- fiscal_period_id, voucher_series) — this setting only controls the UI
+-- default.
+
+ALTER TABLE public.company_settings
+  ADD COLUMN IF NOT EXISTS default_voucher_series text NOT NULL DEFAULT 'A';
+
+-- ADD COLUMN IF NOT EXISTS silently skips the whole column definition
+-- (including NOT NULL) when the column already exists. Re-apply the
+-- constraint explicitly so environments that received the column
+-- out-of-band still end up with the same schema.
+ALTER TABLE public.company_settings
+  ALTER COLUMN default_voucher_series SET DEFAULT 'A';
+
+UPDATE public.company_settings
+  SET default_voucher_series = 'A'
+  WHERE default_voucher_series IS NULL;
+
+ALTER TABLE public.company_settings
+  ALTER COLUMN default_voucher_series SET NOT NULL;
+
+-- Match the Zod validation in lib/api/schemas.ts so DB and app stay in sync.
+ALTER TABLE public.company_settings
+  DROP CONSTRAINT IF EXISTS company_settings_default_voucher_series_check;
+
+ALTER TABLE public.company_settings
+  ADD CONSTRAINT company_settings_default_voucher_series_check
+  CHECK (default_voucher_series ~ '^[A-Z]$');
+
+-- Reload PostgREST schema cache so the column becomes visible to the API
+-- immediately, without waiting for the next automatic reload.
+NOTIFY pgrst, 'reload schema';