Authorities said the suspect approached the woman in an alleyway in the 8400 block of Rosemead Boulevard near Slauson Avenue at about 7 p.m. on Jan. 24.
The suspect threatened the woman and sexually assaulted her, and during the attack, he slashed the victim's cheek with a small knife, said LAPD Det. Joanne Labella.
The man fled southbound down the alley on foot.
The attacker is described as a Hispanic male in his mid-20s, 6 feet tall, 260 pounds, clean-shaven with bushy eyebrows. The suspect also spoke with a Spanish accent.
Authorities said there were surveillance cameras in the area, but because a light was out it was too dark to see anything.
Anyone with information related to the crime is urged to call the L.A. County Sheriff's Pico Rivera Station at (562) 949-2421.