Police were called to the restaurant at 12625 Westheimer around 3:45 p.m.
The restaurant says the robber was wearing a mask when he drew a gun inside the restaurant.
The robber grabbed some cash and took off on foot.
Workers at the McDonald's calmly backed into the manager's office to call police, per company protocol, the restaurant said.
There were no injuries. We do not know if police have any leads on the suspect.