Police spokesman Sgt. Wilfred Williams said a San Francisco Recreation and Park Department crew made the find at about 10:30 a.m. while cleaning out a homeless encampment near the Gleneagles International Golf Course.
The area is at Sunnydale and Persia avenues on the western side of the park.
"As they were driving away from a particular area, they noticed a leg sticking out from underneath a blue tarp," said Williams. He said the find was accompanied by a pungent odor.
The crew then notified police, Williams said.
Police have roped off an approximately 50-square-yard area where units from homicide, crime scene investigation, the crime lab and the medical examiner's office are investigating, according to Williams.
"At this point, we are going to excavate the area to determine if there are more pieces of a body to be found," he said.
"There was only a leg found," Williams said.
Police have not yet determined the age, gender or race of the person, or how long the leg had been there, he said.