A 17-year-old girl said she was stopped in her vehicle at Potomac and Lillian streets when the man climbed inside, according to officials.
The girl said he pulled out a gun and sexually assaulted her before leaving.
The suspect was described as a Hispanic man between 35 and 40 years old. He had dark brown "buzzed" hair and blotchy white spots on his neck and arms.
Anyone with information about the case was urged to call Riverside police at (951) 353-7120.