Officers responded to a report of a shooting at a house in the 100 block of Ottawa Street at about 10:20 p.m.
Police did not say how many shots were fired, but confirmed that six people were inside the home at the time.
Investigators believe the shooting was an isolated incident.
Patrols in the neighborhood have been increased, police said.
Anyone with information about the case is asked to contact the San Mateo Police Department at (650) 522-7650 or the secret witness hotline at (650) 5422-7676.