diff --git a/docs/api/server.md b/docs/api/server.md
index 5286687..fdb4a36 100644
--- a/docs/api/server.md
+++ b/docs/api/server.md
@@ -44,6 +44,7 @@ Instantiates `OAuth2Server` using the supplied model.
 | [options.alwaysIssueNewRefreshToken] | <code>boolean</code> | <code>true</code> | Always revoke the used refresh token and issue a new one for the `refresh_token` grant. |
 | [options.extendedGrantTypes] | <code>object</code> | <code>object</code> | Additional supported grant types. |
 | [options.enablePlainPKCE] | <code>boolean</code> | <code>false</code> | Allow the use of the `plain` code challenge method for PKCE. This is not recommended for production environments. |
+| [options.requirePKCE] | <code>boolean</code> | <code>false</code> | Require PKCE for the `authorization_code` grant: `authorize` rejects requests without a `code_challenge`, and the token exchange rejects authorization codes that were issued without one. Recommended by OAuth 2.1. |
 
 **Example**  
 ```js
diff --git a/docs/guide/pkce.md b/docs/guide/pkce.md
index 3eace68..f216dfd 100644
--- a/docs/guide/pkce.md
+++ b/docs/guide/pkce.md
@@ -29,6 +29,39 @@ Figure 2: Abstract Protocol Flow
 
 See [Section 1 of RFC 7636](https://datatracker.ietf.org/doc/html/rfc7636#section-1.1).
 
+## Requiring PKCE
+
+By default PKCE is *optional*: the library verifies a `code_challenge` when one is
+present (and enforces the
+[RFC 7636 §4.6](https://datatracker.ietf.org/doc/html/rfc7636#section-4.6)
+downgrade protection — a `code_verifier` with no stored challenge is rejected),
+but a client may also complete the `authorization_code` flow without it.
+
+[OAuth 2.1](https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1) makes
+PKCE **mandatory** for the authorization code grant — for all clients, public and
+confidential — and
+[RFC 9700 (OAuth 2.0 Security BCP) §2.1.1](https://www.rfc-editor.org/rfc/rfc9700#section-2.1.1)
+recommends that authorization servers require it, partly to defend against PKCE
+*downgrade* attacks. To enforce this, enable `requirePKCE`:
+
+```js
+const server = new OAuth2Server({
+  model,
+  requirePKCE: true
+})
+```
+
+When enabled:
+
+- the **authorization** endpoint rejects requests without a `code_challenge`
+  (`invalid_request`), so no PKCE-less codes are ever issued; and
+- the **token** endpoint rejects authorization codes that were issued without a
+  `code_challenge` (`invalid_grant`) — covering codes minted before the option
+  was turned on, or through another path.
+
+`requirePKCE` defaults to `false` to preserve backwards compatibility. It is a
+strong candidate to become the default in a future major release.
+
 ## 1. Authorization request
 
 <div id="PKCE#authorizationRequest">
diff --git a/index.d.ts b/index.d.ts
index e67540b..920b47a 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -208,6 +208,11 @@ declare namespace OAuth2Server {
          * Lifetime of generated authorization codes in seconds (default = 5 minutes).
          */
         authorizationCodeLifetime?: number;
+
+        /**
+         * Require PKCE for the authorization code grant: reject authorize requests without a `code_challenge`. Recommended by OAuth 2.1.
+         */
+        requirePKCE?: boolean;
     }
 
     interface TokenOptions {
@@ -240,6 +245,11 @@ declare namespace OAuth2Server {
          * Additional supported grant types.
          */
         extendedGrantTypes?: Record<string, typeof AbstractGrantType>;
+
+        /**
+         * Require PKCE for the authorization code grant: reject token exchanges for codes issued without a `code_challenge`. Recommended by OAuth 2.1.
+         */
+        requirePKCE?: boolean;
     }
 
     /**
diff --git a/lib/grant-types/authorization-code-grant-type.js b/lib/grant-types/authorization-code-grant-type.js
index 56df29e..6e93470 100644
--- a/lib/grant-types/authorization-code-grant-type.js
+++ b/lib/grant-types/authorization-code-grant-type.js
@@ -42,6 +42,8 @@ class AuthorizationCodeGrantType extends AbstractGrantType {
 
     // xxx: plain PKCE is only allowed if explicitly enabled
     this.enablePlainPKCE = options.enablePlainPKCE === true;
+    // when enabled, the authorization code grant requires PKCE
+    this.requirePKCE = options.requirePKCE === true;
   }
 
   /**
@@ -165,6 +167,11 @@ class AuthorizationCodeGrantType extends AbstractGrantType {
       }
     }
     else {
+      if (this.requirePKCE) {
+        // PKCE is required, but the authorization code was not associated with a `code_challenge`.
+        throw new InvalidGrantError('Invalid grant: authorization code was issued without a `code_challenge`');
+      }
+
       if (request.body.code_verifier) {
         // No code challenge but code_verifier was passed in.
         throw new InvalidGrantError('Invalid grant: code verifier is invalid');
diff --git a/lib/handlers/authorize-handler.js b/lib/handlers/authorize-handler.js
index c5f0db1..71c9623 100644
--- a/lib/handlers/authorize-handler.js
+++ b/lib/handlers/authorize-handler.js
@@ -63,6 +63,7 @@ class AuthorizeHandler {
     this.authenticateHandler = options.authenticateHandler || new AuthenticateHandler(options);
     this.authorizationCodeLifetime = options.authorizationCodeLifetime;
     this.enablePlainPKCE = options.enablePlainPKCE === true;
+    this.requirePKCE = options.requirePKCE === true;
     this.model = options.model;
   }
 
@@ -101,6 +102,11 @@ class AuthorizeHandler {
       const ResponseType = this.getResponseType(request);
       const codeChallenge = this.getCodeChallenge(request);
       const codeChallengeMethod = this.getCodeChallengeMethod(request);
+
+      if (this.requirePKCE && !codeChallenge) {
+        throw new InvalidRequestError('Missing parameter: `code_challenge`');
+      }
+
       const code = await this.saveAuthorizationCode(
         authorizationCode,
         expiresAt,
diff --git a/lib/handlers/token-handler.js b/lib/handlers/token-handler.js
index 3b98307..650bf48 100644
--- a/lib/handlers/token-handler.js
+++ b/lib/handlers/token-handler.js
@@ -62,6 +62,7 @@ class TokenHandler {
     this.requireClientAuthentication = options.requireClientAuthentication || {};
     this.alwaysIssueNewRefreshToken = options.alwaysIssueNewRefreshToken !== false;
     this.enablePlainPKCE = options.enablePlainPKCE === true;
+    this.requirePKCE = options.requirePKCE === true;
   }
 
   /**
@@ -231,7 +232,8 @@ class TokenHandler {
       model: this.model,
       refreshTokenLifetime: refreshTokenLifetime,
       alwaysIssueNewRefreshToken: this.alwaysIssueNewRefreshToken,
-      enablePlainPKCE: this.enablePlainPKCE === true
+      enablePlainPKCE: this.enablePlainPKCE === true,
+      requirePKCE: this.requirePKCE === true
     };
 
     return new Type(options).handle(request, client);
diff --git a/lib/server.js b/lib/server.js
index 320ddea..f5a5a32 100644
--- a/lib/server.js
+++ b/lib/server.js
@@ -45,6 +45,7 @@ class OAuth2Server {
    * @param [options.alwaysIssueNewRefreshToken=true] {boolean=} Always revoke the used refresh token and issue a new one for the `refresh_token` grant.
    * @param [options.extendedGrantTypes=object] {object} Additional supported grant types.
    * @param [options.enablePlainPKCE=false] {boolean} Allow the use of the `plain` code challenge method for PKCE. This is not recommended for production environments.
+   * @param [options.requirePKCE=false] {boolean} Require PKCE for the `authorization_code` grant: `authorize` rejects requests without a `code_challenge`, and the token exchange rejects authorization codes that were issued without one. Recommended by OAuth 2.1.
    *
    * @throws {InvalidArgumentError} if the model is missing
    * @return {OAuth2Server} A new `OAuth2Server` instance.
diff --git a/test/compliance/pkce_test.js b/test/compliance/pkce_test.js
index fa46b8f..a1bd94d 100644
--- a/test/compliance/pkce_test.js
+++ b/test/compliance/pkce_test.js
@@ -671,4 +671,99 @@ describe('PKCE Compliance (RFC 7636)', function () {
       }
     });
   });
+
+  // ==================================================================
+  // requirePKCE option (OAuth 2.1 / RFC 9700 §2.1.1)
+  //
+  // When `requirePKCE` is enabled, the authorization_code grant must use
+  // PKCE: the authorize endpoint rejects requests without a
+  // `code_challenge`, and the token endpoint rejects authorization codes
+  // that were issued without one.
+  // ==================================================================
+  describe('requirePKCE option', function () {
+    function pkceModel () {
+      const baseModel = createModel(db);
+      return {
+        ...baseModel,
+        getAuthorizationCode: async (authorizationCode) => db.authorizationCodes.get(authorizationCode) || null,
+        saveAuthorizationCode: async (code, client, user) => {
+          const doc = { ...code, client, user };
+          db.authorizationCodes.set(code.authorizationCode, doc);
+          return doc;
+        },
+        revokeAuthorizationCode: async (code) => db.authorizationCodes.delete(code.authorizationCode),
+        validateScope: async (user, client, scope) => scope
+      };
+    }
+
+    function requirePKCEServer () {
+      return new OAuth2Server({ requirePKCE: true, authorizationCodeLifetime: 300, model: pkceModel() });
+    }
+
+    function authorizeRequest (extraQuery = {}) {
+      return createRequest({
+        method: 'GET',
+        query: {
+          response_type: 'code',
+          client_id: clientDoc.id,
+          redirect_uri: clientDoc.redirectUris[0],
+          state: 'teststate',
+          scope: 'read',
+          ...extraQuery
+        }
+      });
+    }
+
+    const authenticateHandler = { handle: () => userDoc };
+
+    it('rejects an authorize request without a `code_challenge`', async function () {
+      const server = requirePKCEServer();
+      const response = new Response({ headers: {} });
+      let error = null;
+      try {
+        await server.authorize(authorizeRequest(), response, { authenticateHandler });
+      } catch (e) {
+        error = e;
+      }
+      (error !== null).should.equal(true);
+      error.should.be.an.instanceOf(InvalidRequestError);
+      error.message.should.match(/code_challenge/);
+    });
+
+    it('allows an authorize request that includes a `code_challenge`', async function () {
+      const server = requirePKCEServer();
+      const response = new Response({ headers: {} });
+      const challenge = computeS256Challenge('a'.repeat(43));
+      const code = await server.authorize(
+        authorizeRequest({ code_challenge: challenge, code_challenge_method: 'S256' }),
+        response,
+        { authenticateHandler }
+      );
+      code.codeChallenge.should.equal(challenge);
+    });
+
+    it('rejects a token exchange for a code issued without a `code_challenge`', async function () {
+      const server = requirePKCEServer();
+      const codeValue = 'no-pkce-code-' + Math.random().toString(36).slice(2);
+      db.authorizationCodes.set(codeValue, {
+        authorizationCode: codeValue,
+        expiresAt: new Date(Date.now() + 60000),
+        redirectUri: 'https://client.example/callback',
+        client: clientDoc,
+        user: userDoc,
+        scope: ['read']
+        // intentionally no codeChallenge
+      });
+      const response = new Response();
+      let error = null;
+      try {
+        await server.token(tokenRequest(codeValue), response);
+      } catch (e) {
+        error = e;
+      }
+      (error !== null).should.equal(true);
+      error.should.be.an.instanceOf(InvalidGrantError);
+      error.message.should.equal('Invalid grant: authorization code was issued without a `code_challenge`');
+    });
+  });
 });