The man, who called himself "Q," followed the woman and tried to speak with her several times before grabbing her in the 2700 block of East 76th Street, Chicago police said.
He covered her mouth shortly after 1 a.m. and threatened to kill her if she screamed, police said. He took her to a backyard and then sexually assaulted her.
The man took her cellphone and then left south toward 76th Street.
Police asked anyone with information to call Area Two detectives at (312) 747-8271.