The robbery was reported at 4:23 p.m. Monday near the intersection of Third Street and Newcomb Avenue.
The 43-year-old victim was approached by a man who identified himself as an FBI agent. The suspect asked to see the victim's identification, and when he pulled out his wallet, the suspect punched him in the chest and stole cash from him, according to police.
The robber, described as a man in his 50s, fled with two other men in their 20s, police said.
The victim was not seriously injured and did not need to be taken to a hospital.
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.