The robbery was reported at about 8:40 a.m. near a hotel in the 300 block of South Airport Boulevard.
According to police, the victims had visited a jewelry center in San Francisco on Wednesday and were possibly followed by the suspects, who waited for the merchants to return to their vehicle on Thursday morning.
When the victims entered their vehicle Thursday, two suspects approached the vehicle and smashed the windows with a hammer before grabbing a bag containing several pieces of jewelry from the back seat and fleeing on South Airport Boulevard with two other suspects in a vehicle.
The suspects wore masks, sunglasses and dark clothing, and the vehicle they drove away in is described as a brown Toyota Camry, according to police.
The robbery is similar to two other robberies in the area in the past year in which jewelry merchants were targeted, police said.
Anyone with information about the incident is asked to contact Officer Alvarado with the South San Francisco Police Department at (650) 877-8900.