The suspect broke into the home on Juniper Avenue around 1 a.m. Tuesday morning by removing some window screens and forcing his way inside, according to the Fontana Police Department.
Police say he sexually assaulted another woman at a different home on the same block a little earlier, but was scared off by a family member.
The suspect, who is between 20 and 30 years old, was last seen wearing a black T-shirt and gray shorts. He has a tattoo of the letter "S" on the left side of his neck and another tattoo on his left forearm, authorities say.
Anyone with information about the incidents, surveillance footage or knowledge about who the suspect is is asked to call Fontana police detectives at (909) 350-8170.