The man's name hasn't been released because officials haven't been able to find his family.
A friend of his, another Martinez area transient, found him shortly before 8:30 a.m. between the soccer fields and the skateboarding park at the Martinez Marina, police said.
Contra Costa County Fire Protection District paramedics pronounced him dead at the scene, Martinez police Sgt. Aaron Roth said.
An autopsy conducted Monday concluded that the man died from blunt force trauma due to a fall from a bicycle, a deputy coroner said. His death has been ruled an accident.