The robbery was reported at 12:53 p.m. in the 900 block of Geary Street near Larkin Street.
The victim, a 34-year-old man, was walking out of the dispensary when a suspect pointed a gun at his face and demanded "weed," according to police.
The victim handed over a satchel that contained 1.25 pounds of medical marijuana and cash, police said.
The suspect, a man in his late teens or early 20s, fled in a black pickup truck with dealer plates. He had not been found as of this morning, according to police.
The victim was not injured in the robbery.
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.