The latest incident occurred at Polliwog Park near 18th Street and Redondo Avenue in Manhattan Beach around 2:10 p.m. Friday.
According to Manhattan Beach police Det. Traci Navarrette, the boy walked into the men's restroom alone "where he was accosted by the suspect, who forced him into a restroom stall and fondled his genitals."
The boy fled the restroom and immediately told his parent.
Police believe the incident may be connected to a similar incident in Hermosa Beach.
A 13-year-old boy was sexually assaulted in the bathroom at Valley Park in Hermosa Beach around 8 p.m. Wednesday. The victim walked into the restroom and was "overpowered" by the suspect and sexually assaulted.
"It sounds like he's a clean-cut guy that lurks in the shadows, so we're asking our residents to keep an eye out," said Hermosa Beach police Sgt. Robert Higgins.
Residents were stunned to learn of the incidents.
"I'm horrified about it. I've lived here for about 23 years, and there's a lot of people who go to that park, a lot of moms go with their children, babysitters go with their children and play there" said Diane Norocea of Hermosa Beach.
The suspect was described as a white man, possibly between 30 and 40 years old, with gray hair, standing around 6 feet tall with a medium build. He was last seen wearing a white shirt and dark-colored pants.
Police have stepped up patrols at both parks as investigators work with both victims to come up with composite sketches as quickly as possible.
The investigation is ongoing. Anyone with any information was urged to contact Navarrette at (310) 802-5125.
CNS contributed to this report.