diff --git a/.changeset/wild-pianos-join.md b/.changeset/wild-pianos-join.md
new file mode 100644
index 0000000000..202bbe8a67
--- /dev/null
+++ b/.changeset/wild-pianos-join.md
@@ -0,0 +1,5 @@
+---
+"better-auth": patch
+---
+
+fix(username): `isUsernameAvailable` should run validation
diff --git a/packages/better-auth/src/plugins/username/index.ts b/packages/better-auth/src/plugins/username/index.ts
index 3dfd0c5359..e7df89136d 100644
--- a/packages/better-auth/src/plugins/username/index.ts
+++ b/packages/better-auth/src/plugins/username/index.ts
@@ -268,6 +268,30 @@ export const username = (options?: UsernameOptions) => {
 							message: ERROR_CODES.INVALID_USERNAME,
 						});
 					}
+					const minUsernameLength = options?.minUsernameLength || 3;
+					const maxUsernameLength = options?.maxUsernameLength || 30;
+					if (username.length < minUsernameLength) {
+						throw new APIError("UNPROCESSABLE_ENTITY", {
+							message: ERROR_CODES.USERNAME_TOO_SHORT,
+						});
+					}
+
+					if (username.length > maxUsernameLength) {
+						throw new APIError("UNPROCESSABLE_ENTITY", {
+							message: ERROR_CODES.USERNAME_TOO_LONG,
+						});
+					}
+
+					const validator =
+						options?.usernameValidator || defaultUsernameValidator;
+
+					const valid = await validator(username);
+					if (!valid) {
+						throw new APIError("UNPROCESSABLE_ENTITY", {
+							message: ERROR_CODES.INVALID_USERNAME,
+						});
+					}
+
 					const user = await ctx.context.adapter.findOne<User>({
 						model: "user",
 						where: [