The suspect is described as a white male in his 20s, 5 feet 10 inches tall with a thin build and shoulder-length dirty blonde hair, according to the East Bay Regional Park District Police Department.
Witnesses have seen him wearing a baseball cap and on some occasions riding a BMX-type bicycle.
No violence has been reported, according to police.
A trail user helped ch of the suspect, which park police are circulating to the public.
Police are urging women to walk or run with a partner, remain aware of their surroundings and report any suspicious people to police.