A call came in to police at 1:45 a.m. about a possible dead body at the McNear Peninsula, a privately owned piece of undeveloped land near the Petaluma River and Steamer Landing Park.
Officers arrived and searched the approximately 10-acre area full of wild grasses, police said.
At about 2:30 a.m. an officer found the body, which appears to have been there for several weeks, police said.
Investigators responded to the scene to search the area for evidence once the sun was up, police said.
The coroner's office was assisting in removing the body.
Anyone with information about the body is asked to call Petaluma police at (707) 778-4372 or an anonymous tip line at (707) 781-1200. Tips can be submitted here.