diff --git a/README.md b/README.md
index ca7c4be..d58982e 100644
--- a/README.md
+++ b/README.md
@@ -18,6 +18,7 @@ built-in type stripping.
 | --- | --- | --- |
 | `tckn` | T.C. Kimlik No checksum (11 digits) | — |
 | `vkn` | Vergi Kimlik No checksum (10 digits, GİB algorithm) | — |
+| `tax-id` | Invoice "VKN / TCKN" field: picks the right check by length | `kind` (`vkn` / `tckn`), `holder` (`company` / `individual`) |
 | `iban` | TR format, reserve digit, ISO 13616 mod-97 | bank code, bank name, grouped format |
 | `phone` | `+90`, `0090`, `0…` and free-form input | E.164, national format, mobile / landline / 444 / 850 |
 | `plate` | province 01–81, letter/digit rules | formatted plate, province |
@@ -32,7 +33,7 @@ built-in type stripping.
 git clone https://github.com/bahriy-dev/tr-validator-api.git
 cd tr-validator-api
 npm start          # http://localhost:3000
-npm test           # node:test, 43 tests
+npm test           # node:test, 47 tests
 ```
 
 Or with Docker:
diff --git a/src/openapi.ts b/src/openapi.ts
index f8f525a..687bf0c 100644
--- a/src/openapi.ts
+++ b/src/openapi.ts
@@ -1,4 +1,4 @@
-const types = ['tckn', 'vkn', 'iban', 'phone', 'plate', 'postal-code'];
+const types = ['tckn', 'vkn', 'tax-id', 'iban', 'phone', 'plate', 'postal-code'];
 
 const result = {
   type: 'object',
@@ -15,6 +15,7 @@ const result = {
 const examples: Record<string, string> = {
   tckn: '10000000146',
   vkn: '1234567890',
+  'tax-id': '1234567890',
   iban: 'TR330006100519786457841326',
   phone: '05321234567',
   plate: '34ABC123',
diff --git a/src/validators/index.ts b/src/validators/index.ts
index 953cb75..6c6b47a 100644
--- a/src/validators/index.ts
+++ b/src/validators/index.ts
@@ -1,6 +1,6 @@
 import type { ValidationResult } from '../types.ts';
 import { validateTckn } from './tckn.ts';
-import { validateVkn } from './vkn.ts';
+import { validateVkn, validateTaxId } from './vkn.ts';
 import { validateIban } from './iban.ts';
 import { validatePhone } from './phone.ts';
 import { validatePlate, validatePostalCode } from './address.ts';
@@ -8,6 +8,7 @@ import { validatePlate, validatePostalCode } from './address.ts';
 export const VALIDATORS = {
   tckn: validateTckn,
   vkn: validateVkn,
+  'tax-id': validateTaxId,
   iban: validateIban,
   phone: validatePhone,
   plate: validatePlate,
@@ -21,7 +22,7 @@ export function isValidatorType(t: unknown): t is ValidatorType {
 }
 
 export { validateTckn, completeTckn } from './tckn.ts';
-export { validateVkn, vknCheckDigit } from './vkn.ts';
+export { validateVkn, vknCheckDigit, validateTaxId, detectTaxIdKind } from './vkn.ts';
 export { validateIban, ibanCheckDigits, TR_BANKS } from './iban.ts';
 export { validatePhone } from './phone.ts';
 export { validatePlate, validatePostalCode } from './address.ts';
diff --git a/src/validators/vkn.ts b/src/validators/vkn.ts
index bae3fc0..8e992cf 100644
--- a/src/validators/vkn.ts
+++ b/src/validators/vkn.ts
@@ -1,4 +1,5 @@
 import type { ValidationResult } from '../types.ts';
+import { validateTckn } from './tckn.ts';
 
 /**
  * Vergi Kimlik Numarası (Turkish tax ID, 10 digits) checksum validation,
@@ -39,3 +40,17 @@ export function detectTaxIdKind(value: string): 'tckn' | 'vkn' | null {
   if (/^\d{10}$/.test(v)) return 'vkn';
   return null;
 }
+
+/**
+ * Validates the "VKN / TCKN" field of an invoice: 10 digits means a company tax number,
+ * 11 digits means an individual's national ID (sole proprietors and e-Arşiv invoices use it).
+ */
+export function validateTaxId(input: string): ValidationResult {
+  const value = String(input ?? '').replace(/\s+/g, '');
+  const kind = detectTaxIdKind(value);
+  if (!kind) return { valid: false, value, reason: 'Tax ID must be a 10-digit VKN or an 11-digit TCKN' };
+  const result = kind === 'vkn' ? validateVkn(value) : validateTckn(value);
+  return result.valid
+    ? { ...result, details: { kind, holder: kind === 'vkn' ? 'company' : 'individual' } }
+    : { ...result, details: { kind } };
+}
diff --git a/test/validators.test.ts b/test/validators.test.ts
index 38f14b8..22cff2e 100644
--- a/test/validators.test.ts
+++ b/test/validators.test.ts
@@ -1,7 +1,7 @@
 import { describe, it } from 'node:test';
 import assert from 'node:assert/strict';
 import {
-  validateTckn, completeTckn, validateVkn, vknCheckDigit, validateIban, ibanCheckDigits,
+  validateTckn, completeTckn, validateVkn, vknCheckDigit, validateTaxId, validateIban, ibanCheckDigits,
   validatePhone, validatePlate, validatePostalCode, listProvinces, provinceByCode,
 } from '../src/validators/index.ts';
 
@@ -31,6 +31,23 @@ describe('VKN', () => {
   });
 });
 
+describe('Tax ID (VKN or TCKN)', () => {
+  it('accepts a VKN as a company', () => {
+    const r = validateTaxId('1234567890');
+    assert.equal(r.valid, true);
+    assert.deepEqual(r.details, { kind: 'vkn', holder: 'company' });
+  });
+  it('accepts a TCKN as an individual', () => {
+    assert.deepEqual(validateTaxId('10000000146').details, { kind: 'tckn', holder: 'individual' });
+  });
+  it('reports which check failed', () => {
+    const r = validateTaxId('10000000147');
+    assert.equal(r.valid, false);
+    assert.equal(r.details?.kind, 'tckn');
+  });
+  it('rejects other lengths', () => assert.match(validateTaxId('123').reason!, /10-digit VKN or an 11-digit TCKN/));
+});
+
 describe('IBAN', () => {
   it('accepts a valid TR IBAN with spaces', () => {
     const r = validateIban('TR33 0006 1005 1978 6457 8413 26');