The robbery occurred at about 8:20 p.m. in the 2100 block of Carlmont Drive.
One of the victims told police she was walking with friends when a man approached the group from behind. The suspect asked the victims if they had a cell phone, and one of the girls handed him her phone to use, police said.
After being given the phone, the suspect took out a black handgun, threatened the victims and fled east on Carlmont Drive, police said. No one was injured in the robbery.
The suspect is described as a Hispanic man about 18-30 years old with a dark complexion. He is between 5 feet 5 inches and 5 feet 10 inches tall with a heavy build and dark hair, police said.
Anyone with information about the incident is asked to contact Belmont police at (650) 595-7400 or the police tip line at (650) 598-3000.