The incident occurred near Del Mar Avenue and Hershey Street around 5:15 p.m.
Officials said the victim was on the sidewalk when the suspect rode by on a bicycle and touched the victim inappropriately.
The suspect was described as a Hispanic man, about 25 years old, weighing between 130 to 140 pounds, with short brown hair and a moustache. The suspect's bike was described as a black mountain bike with white lettering.
The investigation is ongoing. Anyone with information on the incident was urged to contact the Los Angeles County Sheriff's Department Temple Station at (626) 285-7171 or the Rosemead Public Safety Center at (626) 569-2292.