The holdup occurred at State Street and West Broadway about 1 p.m., according to San Diego police.
The unarmed bandit pushed the victim while stealing cash from him, then fled to the west along with his cohort in a red car, SDPD public-affairs Lt. Andra Brown said.
The robber with the weapon appeared to be in his mid-20s to early 30s and was wearing black jeans, a white shirt and a red baseball-style cap. The other thief, who looked to be in his late teens to mid-20s, was wearing white pants and a green shirt, the lieutenant said.
The victim reported no injuries.