Some visitors from Switzerland found the body on the beach, which is within the jurisdiction of the federal recreation area, park spokeswoman Alexandra Picavet said.
The Marin County Fire Department received a call at 11:23 a.m. today about a report of a body located at Stinson Beach, a fire dispatcher said.
According to Picavet, the sheriff's office notified park officials about the body at about 12:10 p.m. and the county coroner was called to the scene.
The body was in such a state of decomposition that the sex of the deceased will not be known until an autopsy is performed, Picavet said.
"This is a body that probably has been out there a while," Picavet said.
The federal park police are conducting an investigation, Picavet said.
Stinson Beach and a parking lot next to it are parts of the national recreation area, according to Picavet.