A man stole the ball from the teen at about 3 p.m. during the Giants' afternoon game against the Boston Red Sox, police spokesman Officer Albie Esparza said.
The ball had been hit by Boston shortstop Stephen Drew and the teen snagged it, Esparza said. Drew hit the three-run homer over the right-field wall in the seventh inning of the game at AT&T Park.
Afterward, a man came up from behind the boy, tackled him, pulled the boy's arm behind his back and took the ball, Esparza said.
The suspect is described as a black man between 35 and 40 years old who stands about 5 feet 9 inches tall and weighs roughly 230 pounds. He was wearing a black baseball cap, a black T-shirt and black jeans.
He had not been found as of this morning, according to Esparza.
The boy was treated at the ballpark for pain to his right wrist but did not need to be taken to a hospital, Esparza said.
A video of the home run posted on MLB.com appears to show a scrum for the ball, after which a bald black man in a black jacket stood up and held the ball in the air.
A spokesperson for the Giants was not immediately available to comment on the incident. The Giants lost Wednesday's game to the Red Sox in a 12-1 rout.