It happened around 6:30pm Sunday at a pet store in the 7000 block of Highway 6.
Detectives say the suspect forced his way into the store as an employee stepped out to empty the trash and confronted the store's manager and another employee inside. He held them at gunpoint while demanding money from the store safe.
The suspect fled the scene on foot, heading toward Glen Chase Drive.
No one was injured in the robbery, and it wasn't immediately clear how much cash, if any, the suspect stole.
"It's happening at McDonald's and Denny's and all over town," customer John Borrello said.
It's nothing Charles Perkins wants to see or expects when making a quick stop for dog food.
"People are working. They're working to pay the bills and for a living, and this is what you have got this going on. You're scared to work. There's no excuse for that," he said.
The suspect is described as a tall, thin white male between the ages of 18 and 25. He was wearing a long sleeved black T-shirt and black pants. He wore a black baseball hat and a red blank face mask.
Anyone with information about the suspect's identity in this case and/or location is asked to call Crime Stoppers of Houston at 713-222-TIPS or to submit a tip via the iWatchHarrisCounty phone app.