The carjacking was reported at about 4 a.m. near the intersection of Seventh and Howard streets.
The 38-year-old victim met the suspect, a man in his 20s, at the Power Exchange, a sex club located at 220 Jones St.
After the pair left in the woman's car, a 2006 green BMW 325i, the suspect pulled out a handgun and told the woman to get out of the car, police said.
The woman complied, and the man fled with the car. The woman was not injured during the carjacking, according to police.
The suspect and car have not yet been found. 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.