Surveillance video from Gameworks has uncovered one image of the suspect so far, but the company has multiple cameras inside and investigators are still searching the footage for better angles.
Police say the gunman used the noise and distraction inside to blind-side the two young boys.
"They are playing games, and next thing they know, they are confronted by this guy," said Mark Jamieson with Seattle Police.
The two teens say the gunman threatened them with a pistol and forced them to hand over a phone and a shoulder bag. Then the robber got violent.
"Then allegedly struck one of the kids in the head, and then fled," Jamieson said.
Gameworks staff say the robbery is quite rare.
"It's the first time in over 16 years that this has happened," said Keri Weinbrecht.
Gameworks staff immediately called police, but officers were not able to track down the gunman. The arcade is now working with detectives to develop leads.
"We are very careful. We are a very safe environment," Weinbrecht said. "We do have security cameras, surveillance and security officers on site."
The victim who got smacked in the back of the head was not seriously hurt.