diff --git a/prisma/migrations/20250623205959_add_pkce_to_auth_code/migration.sql b/prisma/migrations/20250623205959_add_pkce_to_auth_code/migration.sql
new file mode 100644
index 0000000..e8a05fc
--- /dev/null
+++ b/prisma/migrations/20250623205959_add_pkce_to_auth_code/migration.sql
@@ -0,0 +1,3 @@
+-- AlterTable
+ALTER TABLE "AuthCode" ADD COLUMN     "codeChallenge" TEXT,
+ADD COLUMN     "codeChallengeMethod" TEXT;
diff --git a/prisma/schema.prisma b/prisma/schema.prisma
index 07ec592..cf06f9e 100644
--- a/prisma/schema.prisma
+++ b/prisma/schema.prisma
@@ -92,6 +92,8 @@ model AuthCode {
   userId     String
   user       User     @relation(fields: [userId], references: [id], onDelete: Cascade)
   redirectUri String
+  codeChallenge String?
+  codeChallengeMethod String?
 
   createdAt DateTime @default(now())
 }
diff --git a/src/app/api/oauth/token/route.ts b/src/app/api/oauth/token/route.ts
index 67f80df..bdd3e3f 100644
--- a/src/app/api/oauth/token/route.ts
+++ b/src/app/api/oauth/token/route.ts
@@ -24,6 +24,7 @@ export async function POST(request: NextRequest) {
   const redirect_uri = formData.get('redirect_uri') as string;
   const client_id = formData.get('client_id') as string;
   const client_secret = formData.get('client_secret') as string | null;
+  const code_verifier = formData.get('code_verifier') as string | undefined;
 
   console.log("Form data:", { grant_type, code, redirect_uri, client_id });
 
@@ -66,20 +67,6 @@ export async function POST(request: NextRequest) {
       });
     }
 
-    if (client.clientSecret && client.clientSecret !== client_secret) {
-      console.log("Invalid client_secret.", { client_id });
-      return NextResponse.json({ error: 'Invalid client' }, { 
-        status: 401,
-        headers: {
-          'Access-Control-Allow-Origin': '*',
-          'Access-Control-Allow-Methods': 'POST, OPTIONS',
-          'Access-Control-Allow-Headers': 'Content-Type, Authorization',
-        }
-      });
-    }
-
-    console.log("Found client:", client.id);
-
     console.log("Finding auth code:", code);
     const authCode = await prisma.authCode.findUnique({ where: { code } });
     if (!authCode || authCode.clientId !== client.id || authCode.redirectUri !== redirect_uri) {
@@ -108,6 +95,54 @@ export async function POST(request: NextRequest) {
     }
     console.log("Auth code is valid.");
 
+    // PKCE validation
+    let pkceValid = false;
+    if (authCode.codeChallenge) {
+      if (!code_verifier) {
+        return NextResponse.json({ error: 'Missing code_verifier for PKCE' }, {
+          status: 400,
+          headers: {
+            'Access-Control-Allow-Origin': '*',
+            'Access-Control-Allow-Methods': 'POST, OPTIONS',
+            'Access-Control-Allow-Headers': 'Content-Type, Authorization',
+          }
+        });
+      }
+      if (authCode.codeChallengeMethod === 'S256') {
+        const hash = require('crypto').createHash('sha256').update(code_verifier).digest();
+        const base64url = hash.toString('base64')
+          .replace(/\+/g, '-')
+          .replace(/\//g, '_')
+          .replace(/=+$/, '');
+        pkceValid = base64url === authCode.codeChallenge;
+      } else if (authCode.codeChallengeMethod === 'plain' || !authCode.codeChallengeMethod) {
+        pkceValid = code_verifier === authCode.codeChallenge;
+      }
+      if (!pkceValid) {
+        return NextResponse.json({ error: 'Invalid code_verifier for PKCE' }, {
+          status: 400,
+          headers: {
+            'Access-Control-Allow-Origin': '*',
+            'Access-Control-Allow-Methods': 'POST, OPTIONS',
+            'Access-Control-Allow-Headers': 'Content-Type, Authorization',
+          }
+        });
+      }
+    }
+
+    // If PKCE is not present or not valid, require client secret for confidential clients
+    if (!authCode.codeChallenge && client.clientSecret && client.clientSecret !== client_secret) {
+      console.log("Invalid client_secret.", { client_id });
+      return NextResponse.json({ error: 'Invalid client' }, { 
+        status: 401,
+        headers: {
+          'Access-Control-Allow-Origin': '*',
+          'Access-Control-Allow-Methods': 'POST, OPTIONS',
+          'Access-Control-Allow-Headers': 'Content-Type, Authorization',
+        }
+      });
+    }
+
     // Delete the auth code so it can't be used again
     console.log("Deleting auth code:", authCode.id);
     await prisma.authCode.delete({ where: { id: authCode.id } });
diff --git a/src/app/oauth/authorize/page.tsx b/src/app/oauth/authorize/page.tsx
index 6454239..66b56c5 100644
--- a/src/app/oauth/authorize/page.tsx
+++ b/src/app/oauth/authorize/page.tsx
@@ -17,6 +17,8 @@ export default async function AuthorizePage({
   const redirectUri = params.redirect_uri as string;
   const responseType = params.response_type as string;
   const state = params.state as string;
+  const code_challenge = params.code_challenge as string | undefined;
+  const code_challenge_method = params.code_challenge_method as string | undefined;
 
   if (!session || !session.user || !session.user.id) {
     const headersList = headers();
@@ -98,6 +100,8 @@ export default async function AuthorizePage({
         clientId: client.id,
         userId: session.user.id,
         redirectUri: redirectUri,
+        codeChallenge: code_challenge,
+        codeChallengeMethod: code_challenge_method,
       },
     });