The attack was reported at 4:58 p.m. Sunday in Sharon Meadow in the eastern end of the park.
The 30-year-old victim was arguing with another man when a third man intervened and hit him in the face with a closed fist, according to police.
The victim fell to the ground and was knocked unconscious. He was taken to the hospital with a head injury that is considered life-threatening, police said.
The suspects, two men believed to be in their 20s, fled and had not been found as of this morning.
Anyone with information about the case is encouraged to call the Police Department's anonymous tip line at (415) 575-4444 or send a tip by text message to TIP411.