Police spokeswoman Norma Eisenman says Alexander hit the biker at about 7:15 a.m. Tuesday in the Mid-City area.
Eisenman didn't immediately have details about the accident but she says Alexander wasn't cited.
She says the boy suffered minor injuries and was in stable condition when he was taken to a hospital for examination.
Alexander played neurotic neighbor George Costanza on "Seinfeld."
Alexander's publicist, Ron Hofmann, says the actor acted responsibly by waiting for police and paramedics to arrive. He says Alexander is grateful that the boy wasn't seriously hurt.