diff --git a/lib/chess/auth/user.ex b/lib/chess/auth/user.ex
index f745bea..9f31d08 100644
--- a/lib/chess/auth/user.ex
+++ b/lib/chess/auth/user.ex
@@ -4,6 +4,7 @@ defmodule Chess.Auth.User do
   use Ecto.Schema
   import Ecto.Changeset
   alias Comeonin.Argon2
+  alias Chess.Repo
 
   schema "users" do
     field :password, :string, virtual: true
@@ -18,6 +19,7 @@ defmodule Chess.Auth.User do
     struct
     |> cast(params, [:username, :password])
     |> validate_required([:username, :password])
+    |> unsafe_validate_unique(:username, Repo)
     |> hash_password()
   end
 
diff --git a/test/chess/auth/user_test.exs b/test/chess/auth/user_test.exs
new file mode 100644
index 0000000..ea65a3d
--- /dev/null
+++ b/test/chess/auth/user_test.exs
@@ -0,0 +1,35 @@
+defmodule Chess.UserTest do
+  use Chess.DataCase
+
+  describe "user" do
+    alias Chess.Auth.User
+
+    @valid_attrs %{username: "zelda", password: "password"}
+    @invalid_attrs %{}
+
+    test "changeset with valid attributes" do
+      changeset = User.changeset(%User{}, @valid_attrs)
+      assert changeset.valid?
+    end
+
+    test "changeset with invalid attributes" do
+      changeset = User.changeset(%User{}, @invalid_attrs)
+      refute changeset.valid?
+    end
+
+    test "username must be unique" do
+      create_user("zelda")
+
+      changeset = User.changeset(%User{}, @valid_attrs)
+      refute changeset.valid?
+    end
+
+    defp create_user(username) do
+      changeset = User.changeset(
+        %User{},
+        %{username: username, password: "password"}
+      )
+      Repo.insert!(changeset)
+    end
+  end
+end