diff --git a/app/routes/users/dialogs/create-user.tsx b/app/routes/users/dialogs/create-user.tsx
index 9794df7..e546ea7 100644
--- a/app/routes/users/dialogs/create-user.tsx
+++ b/app/routes/users/dialogs/create-user.tsx
@@ -3,6 +3,7 @@ import Dialog, { DialogPanel } from "~/components/dialog";
 import Input from "~/components/input";
 import Text from "~/components/text";
 import Title from "~/components/title";
+import { USERNAME_PATTERN, USERNAME_RULE } from "~/utils/user";
 
 interface CreateUserProps {
   isOidc?: boolean;
@@ -23,7 +24,17 @@ export default function CreateUser({ isOidc, isDisabled }: CreateUserProps) {
         </Text>
         <input name="action_id" type="hidden" value="create_user" />
         <div className="flex flex-col gap-4">
-          <Input required label="Username" name="username" placeholder="my-new-user" type="text" />
+          <Input
+            description={USERNAME_RULE}
+            minLength={2}
+            pattern={USERNAME_PATTERN}
+            required
+            title={USERNAME_RULE}
+            label="Username"
+            name="username"
+            placeholder="my-new-user"
+            type="text"
+          />
           <Input label="Display Name" name="display_name" placeholder="John Doe" type="text" />
           <Input label="Email" name="email" placeholder="name@example.com" type="email" />
         </div>
diff --git a/app/routes/users/dialogs/rename-user.tsx b/app/routes/users/dialogs/rename-user.tsx
index 96f8976..bc0560c 100644
--- a/app/routes/users/dialogs/rename-user.tsx
+++ b/app/routes/users/dialogs/rename-user.tsx
@@ -3,6 +3,7 @@ import Input from "~/components/input";
 import Text from "~/components/text";
 import Title from "~/components/title";
 import { User } from "~/types";
+import { USERNAME_PATTERN, USERNAME_RULE } from "~/utils/user";
 
 interface RenameProps {
   user: User;
@@ -10,7 +11,6 @@ interface RenameProps {
   setIsOpen: (isOpen: boolean) => void;
 }
 
-// TODO: Server side validation before submitting
 export default function RenameUser({ user, isOpen, setIsOpen }: RenameProps) {
   return (
     <Dialog isOpen={isOpen} onOpenChange={setIsOpen}>
@@ -24,7 +24,11 @@ export default function RenameUser({ user, isOpen, setIsOpen }: RenameProps) {
         <input name="headscale_user_id" type="hidden" value={user.id} />
         <Input
           defaultValue={user.name}
+          description={USERNAME_RULE}
+          minLength={2}
+          pattern={USERNAME_PATTERN}
           required
+          title={USERNAME_RULE}
           label="Username"
           name="new_name"
           placeholder="my-new-name"
diff --git a/app/routes/users/user-actions.ts b/app/routes/users/user-actions.ts
index 1978951..1ea8cd4 100644
--- a/app/routes/users/user-actions.ts
+++ b/app/routes/users/user-actions.ts
@@ -5,6 +5,7 @@ import { usersResource } from "~/server/headscale/live-store";
 import { isUserPrincipal } from "~/server/web/auth";
 import { Capabilities } from "~/server/web/roles";
 import type { Role } from "~/server/web/roles";
+import { validateUsername } from "~/utils/user";
 
 import type { Route } from "./+types/overview";
 
@@ -42,6 +43,11 @@ export async function userAction({ request, context }: Route.ActionArgs) {
         });
       }
 
+      const nameError = validateUsername(name);
+      if (nameError) {
+        throw data(nameError, { status: 400 });
+      }
+
       await api.users.create({ name, email, displayName });
       await headscaleLiveStore.refresh(usersResource, api);
       return { message: "User created successfully" };
@@ -65,6 +71,11 @@ export async function userAction({ request, context }: Route.ActionArgs) {
         return data({ success: false }, 400);
       }
 
+      const newNameError = validateUsername(newName);
+      if (newNameError) {
+        throw data(newNameError, { status: 400 });
+      }
+
       const users = await api.users.list({ id: headscaleUserId });
       const user = users.find((user) => user.id === headscaleUserId);
       if (!user) {
diff --git a/app/utils/user.ts b/app/utils/user.ts
index 1c7a3ad..aa19dff 100644
--- a/app/utils/user.ts
+++ b/app/utils/user.ts
@@ -7,3 +7,25 @@ export function getUserDisplayName(user: User): string {
 
   return user.name || user.displayName || user.email || user.id;
 }
+
+// Mirrors Headscale's `util.ValidateUsername` (hscontrol/util/dns.go): a name has
+// to start with a letter and may only contain letters, numbers, `-`, `.`, `_` and
+// at most one `@` (`\p{L}`/`\p{Nd}` match Go's `unicode.IsLetter`/`IsDigit`).
+// The trailing `@` is ours: Headscale strips it when resolving a
+// username in an ACL policy, so a user whose name ends in `@` is created fine but
+// can never be matched by a rule.
+export const USERNAME_PATTERN = String.raw`\p{L}[\p{L}\p{Nd}._\-]*(@[\p{L}\p{Nd}._\-]+)?`;
+
+export const USERNAME_RULE =
+  "Usernames must be at least 2 characters, start with a letter, and contain only " +
+  "letters, numbers, dots, dashes and underscores, with at most one @ that cannot " +
+  "be the last character.";
+
+const usernameRegex = new RegExp(`^${USERNAME_PATTERN}$`, "u");
+
+// Returns an error message when the username is not usable in Headscale.
+export function validateUsername(name: string): string | undefined {
+  if (name.length < 2 || !usernameRegex.test(name)) {
+    return USERNAME_RULE;
+  }
+}
diff --git a/tests/unit/utils/user.test.ts b/tests/unit/utils/user.test.ts
index a11dbd0..52d7ba8 100644
--- a/tests/unit/utils/user.test.ts
+++ b/tests/unit/utils/user.test.ts
@@ -1,7 +1,7 @@
 import { describe, expect, test } from "vitest";
 
 import type { User } from "~/types/User";
-import { getUserDisplayName } from "~/utils/user";
+import { getUserDisplayName, USERNAME_PATTERN, validateUsername } from "~/utils/user";
 
 const makeUser = (overrides: Partial<User>): User => ({
   id: "default-id",
@@ -56,3 +56,30 @@ describe("getUserDisplayName", () => {
     expect(getUserDisplayName(user)).toBe("John Doe");
   });
 });
+
+describe("validateUsername", () => {
+  test.each(["ab", "john_doe", "user-1.name", "alice@example.com", "josé"])(
+    "accepts %s",
+    (name) => {
+      expect(validateUsername(name)).toBeUndefined();
+    },
+  );
+
+  // "lm@" is the case from #502: Headscale creates it, but policies strip the
+  // trailing "@" so no rule can ever match the user.
+  // `ab²`/`abⅫ` are Nl/No, which Go's unicode.IsDigit rejects.
+  test.each(["", "a", "1abc", "@abc", "lm@", "a@b@c", "bad name", "bad/name", "ab²", "abⅫ"])(
+    "rejects %s",
+    (name) => {
+      expect(validateUsername(name)).toBeTypeOf("string");
+    },
+  );
+});
+
+describe("USERNAME_PATTERN", () => {
+  test("is a valid HTML pattern (unicode sets mode)", () => {
+    const regex = new RegExp(`^${USERNAME_PATTERN}$`, "v");
+    expect(regex.test("alice@example.com")).toBe(true);
+    expect(regex.test("lm@")).toBe(false);
+  });
+});