According to investigators, it happened on Sunday, July 31, around 1:30 p.m. in the 1100 block of East 4th Street near Alamitos Beach.
Police say the suspect followed the victim for several blocks.
WATCH NOW ABC7 Los Angeles 24x7 Streaming channel
"While following the victim, the suspect propositioned him and then grabbed his lower body," read a statement issued by LBPD.
Some sort of altercation ensued in which the suspect hit the victim before taking off, investigators said.
Detectives released video and a photograph of the suspect from surveillance footage of the incident.
The suspect is being described as a white or Hispanic man with a light complexion, possibly between the ages of 18 and 25.
Police said he's 5 feet 10 inches or 5 feet 11 inches tall.
The suspect reportedly has a thin build, short to medium hair in a "comb over" style and a light-colored mustache.
He's seen wearing a blue Dodgers T-Shirt with light-colored pants and black sandals.
Police said the suspect may have attended a party in the area before the assault.
Anyone with information is urged to contact Det. Monica Moore at 562-570-5514 or Det. William Neal at 562-570-5513.
You can also submit an anonymous tip by calling LA Crime Stoppers at 1-800-222-TIPS (8477) or visiting www.lacrimestoppers.org.