diff --git a/py-be/tests/test_reg.py b/py-be/tests/test_reg.py
new file mode 100644
index 00000000..f5123fbe
--- /dev/null
+++ b/py-be/tests/test_reg.py
@@ -0,0 +1,75 @@
+"""Integration tests for the POST /reg registration endpoint."""
+
+from app.models.base import User
+
+
+def test_register_user_success(db_session, client):
+    payload = {"username": "newuser", "email": "newuser@example.com", "password": "secret123"}
+    res = client.post("/reg", json=payload)
+    assert res.status_code == 201
+    data = res.json()
+    assert "id" in data and isinstance(data["id"], int)
+
+    # Ensure DB persistence
+    user = db_session.query(User).filter_by(id=data["id"]).first()
+    assert user is not None
+    assert user.username == "newuser"
+    assert user.email == "newuser@example.com"
+
+
+def test_register_user_missing_fields(client):
+    # Missing password
+    res = client.post("/reg", json={"username": "x", "email": "x@example.com"})
+    assert res.status_code == 422
+
+    # Missing email
+    res = client.post("/reg", json={"username": "x", "password": "p"})
+    assert res.status_code == 422
+
+    # Missing username
+    res = client.post("/reg", json={"email": "x@example.com", "password": "p"})
+    assert res.status_code == 422
+
+
+def test_register_user_invalid_email(client):
+    res = client.post(
+        "/reg", json={"username": "bademail", "email": "not-an-email", "password": "p"}
+    )
+    # Pydantic EmailStr should reject invalid email with 422
+    assert res.status_code == 422
+
+
+def test_register_user_duplicate_username(db_session, client):
+    # Create initial user
+    res = client.post(
+        "/reg",
+        json={"username": "dupuser", "email": "dup1@example.com", "password": "p"},
+    )
+    assert res.status_code == 201
+
+    # Attempt duplicate username with different email
+    res = client.post(
+        "/reg",
+        json={"username": "dupuser", "email": "dup2@example.com", "password": "p"},
+    )
+    assert res.status_code == 400
+    assert res.json()["detail"] == "User with this username or email already exists"
+
+
+def test_register_user_duplicate_email(db_session, client):
+    # Create initial user
+    res = client.post(
+        "/reg",
+        json={"username": "u1", "email": "dup@example.com", "password": "p"},
+    )
+    assert res.status_code == 201
+
+    # Attempt duplicate email with different username
+    res = client.post(
+        "/reg",
+        json={"username": "u2", "email": "dup@example.com", "password": "p"},
+    )
+    assert res.status_code == 400
+    assert res.json()["detail"] == "User with this username or email already exists"
+
+