Witnesses say several people were in the Blue Taco on Buffalo Gap Road when the suspect walked in through the back door at about 6:30 Wednesday evening.
He went into the office, showed the manager a gun and told him to open the safe.
The manager says the robber got away with a couple of bags of money.
Customers who were eating while the robbery happened say they didn't know what happened until the suspect was gone.
"Nobody in the whole place knew anything, because had actually walked around the building," said Tracy Walker, who was in the restaurant during the robbery. "We saw him through the window and he came in the back door like he was supposed to be there, and the employees even didn't know he wasn't supposed to be there until after he walked back out of the back room."
Employees saw the man drive away in a red pickup truck, then wrote the license plate number down and gave it to police.