The attack happened Sunday around 9 p.m., police say.
Investigators say the suspect and teen were riding the same Metro train from a station in downtown Long Beach, but were not together. The train was headed northbound to the Los Angeles area.
The teen then got off the train and took a bus The suspect, described as a man around 35 years old, followed her onto the bus.
She got off the bus near Vernon Avenue and San Pedro Street. He again followed her.
Police say that's when the suspect went up to the teen from behind and grabbed her by the neck.
"The suspect forced the victim to a parking lot of an apartment building where he threatened to kill her and then raped her," the LAPD said.
Police say the suspect is a Black male believed to be around 35 years old, standing about 6 feet 2 inches tall and weighing 200 pounds with a muscular build. He has black dreadlocks and brown eyes and was last seen wearing a white shirt with black pants.
Anyone with information is asked to call the LAPD Juvenile Division at 213-486-0570.
You can also call LA Regional Crime Stoppers at 1-800-222-TIPS (800-222-8477) or visit www.lacrimestoppers.org.