The incident happened at about 2:30 a.m. Friday at Rieber Hall at 310 De Neve Drive. The suspect grabbed the victim from behind and attempted to sexually assault her.
Two witnesses intervened and the suspect ran off.
Police say the suspect groped a second victim at another residence hall before getting away.
The suspect is described as between 20 and 25 years old, about 5 feet 1 inch to 6 feet 1 inch tall, weighing between 180 and 200 pounds with a thin mustache. He was last seen wearing a blue tank top, blue jeans and tennis shoes.
If you have any information about the suspect, you're urged to contact the UCLA Police Department at (310) 825-1491.