diff --git a/library/CHANGELOG.md b/library/CHANGELOG.md
index f2bde787a..e17766040 100644
--- a/library/CHANGELOG.md
+++ b/library/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 All notable changes to the library will be documented in this file.
 
+## vX.X.X (Month DD, YYYY)
+
+- Add `base64` action to validate Base64 strings (pull request #644)
+
 ## v0.36.0 (July 05, 2024)
 
 - Add `normalize` action to normalize strings (issue #691)
diff --git a/library/src/actions/base64/base64.test-d.ts b/library/src/actions/base64/base64.test-d.ts
new file mode 100644
index 000000000..7c0f628e6
--- /dev/null
+++ b/library/src/actions/base64/base64.test-d.ts
@@ -0,0 +1,43 @@
+import { describe, expectTypeOf, test } from 'vitest';
+import type { InferInput, InferIssue, InferOutput } from '../../types/index.ts';
+import { base64, type Base64Action, type Base64Issue } from './base64.ts';
+
+describe('base64', () => {
+  describe('should return action object', () => {
+    test('with undefined message', () => {
+      type Action = Base64Action<string, undefined>;
+      expectTypeOf(base64<string>()).toEqualTypeOf<Action>();
+      expectTypeOf(
+        base64<string, undefined>(undefined)
+      ).toEqualTypeOf<Action>();
+    });
+
+    test('with string message', () => {
+      expectTypeOf(base64<string, 'message'>('message')).toEqualTypeOf<
+        Base64Action<string, 'message'>
+      >();
+    });
+
+    test('with function message', () => {
+      expectTypeOf(base64<string, () => string>(() => 'message')).toEqualTypeOf<
+        Base64Action<string, () => string>
+      >();
+    });
+  });
+
+  describe('should infer correct types', () => {
+    type Action = Base64Action<string, undefined>;
+
+    test('of input', () => {
+      expectTypeOf<InferInput<Action>>().toEqualTypeOf<string>();
+    });
+
+    test('of output', () => {
+      expectTypeOf<InferOutput<Action>>().toEqualTypeOf<string>();
+    });
+
+    test('of issue', () => {
+      expectTypeOf<InferIssue<Action>>().toEqualTypeOf<Base64Issue<string>>();
+    });
+  });
+});
diff --git a/library/src/actions/base64/base64.test.ts b/library/src/actions/base64/base64.test.ts
new file mode 100644
index 000000000..0e7dfbde1
--- /dev/null
+++ b/library/src/actions/base64/base64.test.ts
@@ -0,0 +1,134 @@
+import { describe, expect, test } from 'vitest';
+import { BASE64_REGEX } from '../../regex.ts';
+import { expectActionIssue, expectNoActionIssue } from '../../vitest/index.ts';
+import { base64, type Base64Action, type Base64Issue } from './base64.ts';
+
+describe('base64', () => {
+  describe('should return action object', () => {
+    const baseAction: Omit<Base64Action<string, never>, 'message'> = {
+      kind: 'validation',
+      type: 'base64',
+      reference: base64,
+      expects: null,
+      requirement: BASE64_REGEX,
+      async: false,
+      _run: expect.any(Function),
+    };
+
+    test('with undefined message', () => {
+      const action: Base64Action<string, undefined> = {
+        ...baseAction,
+        message: undefined,
+      };
+      expect(base64()).toStrictEqual(action);
+      expect(base64(undefined)).toStrictEqual(action);
+    });
+
+    test('with string message', () => {
+      expect(base64('message')).toStrictEqual({
+        ...baseAction,
+        message: 'message',
+      } satisfies Base64Action<string, string>);
+    });
+
+    test('with function message', () => {
+      const message = () => 'message';
+      expect(base64(message)).toStrictEqual({
+        ...baseAction,
+        message,
+      } satisfies Base64Action<string, typeof message>);
+    });
+  });
+
+  describe('should return dataset without issues', () => {
+    const action = base64();
+
+    test('for untyped inputs', () => {
+      expect(action._run({ typed: false, value: null }, {})).toStrictEqual({
+        typed: false,
+        value: null,
+      });
+    });
+
+    test('for empty string', () => {
+      expectNoActionIssue(action, ['']);
+    });
+
+    test('for Base64 strings', () => {
+      expectNoActionIssue(action, [
+        // Test vectors from https://datatracker.ietf.org/doc/html/rfc4648#section-10
+        '', // ''
+        'Zg==', // 'f'
+        'Zm8=', // 'fo'
+        'Zm9v', // 'foo'
+        'Zm9vYg==', // 'foob'
+        'Zm9vYmE=', // 'fooba'
+        'Zm9vYmFy', // 'foobar'
+
+        // Other custom tests
+        'dmFsaWJvdA==', // 'valibot'
+        'SGVsbG8sIEkgYW0gVmFsaWJvdCBhbmQgSSB3b3VsZCBsaWtlIHRvIGhlbHAgeW91IHZhbGlkYXRlIGRhdGEgZWFzaWx5IHVzaW5nIGEgc2NoZW1hLg==', // 'Hello, I am Valibot and I would like to help you validate data easily using a schema.'
+        '8J+Mrg==', // '🌮'
+      ]);
+    });
+  });
+
+  describe('should return dataset with issues', () => {
+    const action = base64('message');
+    const baseIssue: Omit<Base64Issue<string>, 'input' | 'received'> = {
+      kind: 'validation',
+      type: 'base64',
+      expected: null,
+      message: 'message',
+      requirement: BASE64_REGEX,
+    };
+
+    test('for blank strings', () => {
+      expectActionIssue(action, baseIssue, [' ', '\n']);
+    });
+
+    test('for invalid chars', () => {
+      expectActionIssue(action, baseIssue, [
+        'foo`', // `
+        'foo~', // ~
+        'foo!', // !
+        'foo@', // @
+        'foo#', // #
+        'foo$', // $
+        'foo%', // %
+        'foo^', // ^
+        'foo&', // &
+        'foo*', // *
+        'foo(', // (
+        'foo)', // )
+        'foo-', // -
+        'foo_', // _
+        'foo[', // [
+        'foo]', // ]
+        'foo{', // {
+        'foo}', // }
+        'foo\\', // \
+        'foo|', // |
+        'foo;', // ;
+        'foo:', // :
+        "foo'", // '
+        'foo"', // "
+        'foo,', // ,
+        'foo.', // .
+        'foo<', // <
+        'foo>', // >
+        'foo?', // ?
+      ]);
+    });
+
+    test('for invalid padding', () => {
+      expectActionIssue(action, baseIssue, [
+        'dmFsaWJvdA', // == missing
+        'dmFsaWJvdA=', // = missing
+        'dmFsaWJvdA===', // = extra
+        'Zm9vYmE', // = missing
+        'Zm9vYmE==', // = extra
+      ]);
+    });
+  });
+});
diff --git a/library/src/actions/base64/base64.ts b/library/src/actions/base64/base64.ts
new file mode 100644
index 000000000..26f4fb1f8
--- /dev/null
+++ b/library/src/actions/base64/base64.ts
@@ -0,0 +1,105 @@
+import { BASE64_REGEX } from '../../regex.ts';
+import type {
+  BaseIssue,
+  BaseValidation,
+  Dataset,
+  ErrorMessage,
+} from '../../types/index.ts';
+import { _addIssue } from '../../utils/index.ts';
+
+/**
+ * Base64 issue type.
+ */
+export interface Base64Issue<TInput extends string> extends BaseIssue<TInput> {
+  /**
+   * The issue kind.
+   */
+  readonly kind: 'validation';
+  /**
+   * The issue type.
+   */
+  readonly type: 'base64';
+  /**
+   * The expected property.
+   */
+  readonly expected: null;
+  /**
+   * The received property.
+   */
+  readonly received: `"${string}"`;
+  /**
+   * The Base64 regex.
+   */
+  readonly requirement: RegExp;
+}
+
+/**
+ * Base64 action type.
+ */
+export interface Base64Action<
+  TInput extends string,
+  TMessage extends ErrorMessage<Base64Issue<TInput>> | undefined,
+> extends BaseValidation<TInput, TInput, Base64Issue<TInput>> {
+  /**
+   * The action type.
+   */
+  readonly type: 'base64';
+  /**
+   * The action reference.
+   */
+  readonly reference: typeof base64;
+  /**
+   * The expected property.
+   */
+  readonly expects: null;
+  /**
+   * The Base64 regex.
+   */
+  readonly requirement: RegExp;
+  /**
+   * The error message.
+   */
+  readonly message: TMessage;
+}
+
+/**
+ * Creates a [Base64](https://en.wikipedia.org/wiki/Base64) validation action.
+ *
+ * @returns A Base64 action.
+ */
+export function base64<TInput extends string>(): Base64Action<
+  TInput,
+  undefined
+>;
+
+/**
+ * Creates a [Base64](https://en.wikipedia.org/wiki/Base64) validation action.
+ *
+ * @param message The error message.
+ *
+ * @returns A Base64 action.
+ */
+export function base64<
+  TInput extends string,
+  const TMessage extends ErrorMessage<Base64Issue<TInput>> | undefined,
+>(message: TMessage): Base64Action<TInput, TMessage>;
+
+export function base64(
+  message?: ErrorMessage<Base64Issue<string>>
+): Base64Action<string, ErrorMessage<Base64Issue<string>> | undefined> {
+  return {
+    kind: 'validation',
+    type: 'base64',
+    reference: base64,
+    async: false,
+    expects: null,
+    requirement: BASE64_REGEX,
+    message,
+    _run(dataset, config) {
+      if (dataset.typed && !this.requirement.test(dataset.value)) {
+        _addIssue(this, 'Base64', dataset, config);
+      }
+      return dataset as Dataset<string, Base64Issue<string>>;
+    },
+  };
+}
diff --git a/library/src/actions/base64/index.ts b/library/src/actions/base64/index.ts
new file mode 100644
index 000000000..51989b3c7
--- /dev/null
+++ b/library/src/actions/base64/index.ts
@@ -0,0 +1 @@
+export * from './base64.ts';
diff --git a/library/src/actions/index.ts b/library/src/actions/index.ts
index 971764bf3..7e0f1311e 100644
--- a/library/src/actions/index.ts
+++ b/library/src/actions/index.ts
@@ -1,4 +1,5 @@
 export * from './await/index.ts';
+export * from './base64/index.ts';
 export * from './bic/index.ts';
 export * from './brand/index.ts';
 export * from './bytes/index.ts';
diff --git a/library/src/regex.ts b/library/src/regex.ts
index 60753a389..046c6533b 100644
--- a/library/src/regex.ts
+++ b/library/src/regex.ts
@@ -1,7 +1,13 @@
+/**
+ * [Base64](https://en.wikipedia.org/wiki/Base64) regex.
+ */
+export const BASE64_REGEX =
+  /^(?:[\da-z+/]{4})*(?:[\da-z+/]{2}==|[\da-z+/]{3}=)?$/iu;
+
 /**
  * [BIC](https://en.wikipedia.org/wiki/ISO_9362) regex.
  */
-export const BIC_REGEX: RegExp = /^[A-Z]{6}(?!00)[A-Z\d]{2}(?:[A-Z\d]{3})?$/u;
+export const BIC_REGEX: RegExp = /^[A-Z]{6}(?!00)[\dA-Z]{2}(?:[\dA-Z]{3})?$/u;
 
 /**
  * [Cuid2](https://github.com/paralleldrive/cuid2) regex.
diff --git a/website/src/routes/api/(actions)/base64/index.mdx b/website/src/routes/api/(actions)/base64/index.mdx
new file mode 100644
index 000000000..723f55361
--- /dev/null
+++ b/website/src/routes/api/(actions)/base64/index.mdx
@@ -0,0 +1,10 @@
+---
+title: base64
+source: /actions/base64/base64.ts
+contributors:
+  - morinokami
+---
+
+# base64
+
+> The content of this page is not yet ready. Until then, please use the [source code](https://github.com/fabian-hiller/valibot/blob/main/library/src/actions/base64/base64.ts) or take a look at [issue #287](https://github.com/fabian-hiller/valibot/issues/287) to help us extend the API reference.
diff --git a/website/src/routes/api/(async)/customAsync/index.mdx b/website/src/routes/api/(async)/customAsync/index.mdx
index 75b241d0d..2ca75d7ae 100644
--- a/website/src/routes/api/(async)/customAsync/index.mdx
+++ b/website/src/routes/api/(async)/customAsync/index.mdx
@@ -106,7 +106,7 @@ The following APIs can be combined with `customAsync`.
 
 <ApiList
   items={[
-    'bic',
+    'base64',
     'brand',
     'bytes',
     'check',
@@ -182,7 +182,8 @@ The following APIs can be combined with `customAsync`.
     'url',
     'uuid',
     'value',
-  ]}
+
+]}
 />
 
 ### Utils
diff --git a/website/src/routes/api/(methods)/config/index.mdx b/website/src/routes/api/(methods)/config/index.mdx
index 00701de72..329481034 100644
--- a/website/src/routes/api/(methods)/config/index.mdx
+++ b/website/src/routes/api/(methods)/config/index.mdx
@@ -149,6 +149,7 @@ The following APIs can be combined with `config`.
 
 <ApiList
   items={[
+    'base64',
     'bic',
     'brand',
     'bytes',
diff --git a/website/src/routes/api/(methods)/fallback/index.mdx b/website/src/routes/api/(methods)/fallback/index.mdx
index 645759eb3..f1c299fc7 100644
--- a/website/src/routes/api/(methods)/fallback/index.mdx
+++ b/website/src/routes/api/(methods)/fallback/index.mdx
@@ -146,6 +146,7 @@ The following APIs can be combined with `fallback`.
 
 <ApiList
   items={[
+    'base64',
     'bic',
     'brand',
     'bytes',
diff --git a/website/src/routes/api/(methods)/pipe/index.mdx b/website/src/routes/api/(methods)/pipe/index.mdx
index d295f8a9d..4c85eac88 100644
--- a/website/src/routes/api/(methods)/pipe/index.mdx
+++ b/website/src/routes/api/(methods)/pipe/index.mdx
@@ -147,6 +147,7 @@ The following APIs can be combined with `pipe`.
 
 <ApiList
   items={[
+    'base64',
     'bic',
     'brand',
     'bytes',
diff --git a/website/src/routes/api/(schemas)/any/index.mdx b/website/src/routes/api/(schemas)/any/index.mdx
index 107be08e2..3baca48c0 100644
--- a/website/src/routes/api/(schemas)/any/index.mdx
+++ b/website/src/routes/api/(schemas)/any/index.mdx
@@ -79,6 +79,7 @@ The following APIs can be combined with `any`.
 
 <ApiList
   items={[
+    'base64',
     'bic',
     'brand',
     'bytes',
diff --git a/website/src/routes/api/(schemas)/custom/index.mdx b/website/src/routes/api/(schemas)/custom/index.mdx
index 14c9ac665..bc999910f 100644
--- a/website/src/routes/api/(schemas)/custom/index.mdx
+++ b/website/src/routes/api/(schemas)/custom/index.mdx
@@ -108,6 +108,7 @@ The following APIs can be combined with `custom`.
 
 <ApiList
   items={[
+    'base64',
     'bic',
     'brand',
     'bytes',
diff --git a/website/src/routes/api/(schemas)/intersect/index.mdx b/website/src/routes/api/(schemas)/intersect/index.mdx
index 50a39a1ea..3af0b0bb7 100644
--- a/website/src/routes/api/(schemas)/intersect/index.mdx
+++ b/website/src/routes/api/(schemas)/intersect/index.mdx
@@ -131,6 +131,7 @@ The following APIs can be combined with `intersect`.
 
 <ApiList
   items={[
+    'base64',
     'bic',
     'brand',
     'bytes',
diff --git a/website/src/routes/api/(schemas)/lazy/index.mdx b/website/src/routes/api/(schemas)/lazy/index.mdx
index 12ce8eb07..207eb2227 100644
--- a/website/src/routes/api/(schemas)/lazy/index.mdx
+++ b/website/src/routes/api/(schemas)/lazy/index.mdx
@@ -166,6 +166,7 @@ The following APIs can be combined with `lazy`.
 
 <ApiList
   items={[
+    'base64',
     'bic',
     'brand',
     'bytes',
diff --git a/website/src/routes/api/(schemas)/string/index.mdx b/website/src/routes/api/(schemas)/string/index.mdx
index 82ae0742d..06c45f19d 100644
--- a/website/src/routes/api/(schemas)/string/index.mdx
+++ b/website/src/routes/api/(schemas)/string/index.mdx
@@ -132,6 +132,7 @@ The following APIs can be combined with `string`.
 
 <ApiList
   items={[
+    'base64',
     'bic',
     'brand',
     'bytes',
diff --git a/website/src/routes/api/(schemas)/union/index.mdx b/website/src/routes/api/(schemas)/union/index.mdx
index ef23871fd..1f47fb5ce 100644
--- a/website/src/routes/api/(schemas)/union/index.mdx
+++ b/website/src/routes/api/(schemas)/union/index.mdx
@@ -147,6 +147,7 @@ The following APIs can be combined with `union`.
 
 <ApiList
   items={[
+    'base64',
     'bic',
     'brand',
     'bytes',
diff --git a/website/src/routes/api/(schemas)/unknown/index.mdx b/website/src/routes/api/(schemas)/unknown/index.mdx
index 7e1747d16..786b67f0d 100644
--- a/website/src/routes/api/(schemas)/unknown/index.mdx
+++ b/website/src/routes/api/(schemas)/unknown/index.mdx
@@ -79,6 +79,7 @@ The following APIs can be combined with `unknwon`.
 
 <ApiList
   items={[
+    'base64',
     'bic',
     'brand',
     'bytes',
diff --git a/website/src/routes/api/menu.md b/website/src/routes/api/menu.md
index 391cbcf22..cbbf8a9d5 100644
--- a/website/src/routes/api/menu.md
+++ b/website/src/routes/api/menu.md
@@ -72,6 +72,7 @@
 
 ## Actions
 
+- [base64](/api/base64/)
 - [bic](/api/bic/)
 - [brand](/api/brand/)
 - [bytes](/api/bytes/)
diff --git a/website/src/routes/guides/(main-concepts)/pipelines/index.mdx b/website/src/routes/guides/(main-concepts)/pipelines/index.mdx
index 4ead019c0..9b9685208 100644
--- a/website/src/routes/guides/(main-concepts)/pipelines/index.mdx
+++ b/website/src/routes/guides/(main-concepts)/pipelines/index.mdx
@@ -45,6 +45,7 @@ Pipeline validation actions examine the input and, if the input does not meet a
 <ApiList
   label="Validation actions"
   items={[
+    'base64',
     'bic',
     'bytes',
     'check',