diff --git a/app/api/users/route.ts b/app/api/users/route.ts
index 07afa848..c56a6f13 100644
--- a/app/api/users/route.ts
+++ b/app/api/users/route.ts
@@ -175,6 +175,24 @@ async function getAdminToken() {
   }
 }
 
+// Validate username according to Keycloak requirements
+function validateUsername(username: string): { isValid: boolean; error?: string } {
+  // Keycloak username requirements:
+  // - Only alphanumeric characters, dots (.), hyphens (-), and underscores (_)
+  // - Must start with a letter or number
+  // - Must be between 3 and 255 characters
+  const usernameRegex = /^[a-zA-Z0-9][a-zA-Z0-9._-]{2,254}$/;
+  
+  if (!usernameRegex.test(username)) {
+    return {
+      isValid: false,
+      error: "Le nom d'utilisateur doit commencer par une lettre ou un chiffre, ne contenir que des lettres, chiffres, points, tirets et underscores, et faire entre 3 et 255 caractères"
+    };
+  }
+  
+  return { isValid: true };
+}
+
 export async function POST(req: Request) {
   const session = await getServerSession(authOptions);
 
@@ -186,6 +204,15 @@ export async function POST(req: Request) {
     const data = await req.json();
     console.log("Creating user:", data);
 
+    // Validate username
+    const usernameValidation = validateUsername(data.username);
+    if (!usernameValidation.isValid) {
+      return NextResponse.json(
+        { error: usernameValidation.error },
+        { status: 400 }
+      );
+    }
+
     const token = await getAdminToken();
     if (!token) {
       return NextResponse.json({ error: "Erreur d'authentification" }, { status: 401 });