Five other suspects involved in the robbery at the crime-ridden Capitol Hill park got away, and police are looking for them.
The incident unfolded at around 1 a.m. Friday.
Two victims were hanging out in the park near the baseball fields when a group of four or five men and one woman cornered them in the field's dugout.
The 17-year-old suspect then pulled out a handgun, pointed it at the victims and demanded their wallets, said Drew Fowler of the Seattle police.
As one of the victims handed over his wallet and cellphone, the suspect fired one shot, grazing the victim's head, Fowler said.
Officers patrolling near Cal Anderson heard the gunshot and rushed to the scene, where they saw the group of suspects running through the park. The 17-year-old broke off from the group and led police on a brief foot chase before officers took him into custody.
The teen suspect was still carrying the victim's wallet and cellphone when officers caught up to him, Fowler said. Police also found his handgun in some bushes where he ditched it near the park.
Medics treated the injured victim's injuries - a flesh wound along the side of his head. He is expected to fully recover. Officers also returned the stolen cellphone and wallet to the man.
Officers booked the teen suspect into the King County Youth Service Center for investigation of robbery.
The armed robbery was the latest in a series of violent crimes at the park, which has become a hangout for homeless people with mental health issues and drug problems.
A pilot plan to keep the lights on at the park all night long to try to cut down on crime was nixed last year after neighbors complained.