The fire was reported at 3:12 a.m. at 408 N. Bayshore Blvd. Firefighters arrived to find a two-story apartment building with flames coming from the second floor, San Mateo fire Battalion Chief Mike Keefe said.
Residents were evacuated from both levels of the building, and the fire was extinguished at 3:39 a.m., Keefe said.
Three of the six apartments in the building were damaged by the fire, while two apartments on the lower level sustained water damage. The blaze caused an estimated $750,000 in damage - $675,000 to the structure and $75,000 to its contents, Keefe said.
The fire displaced 18 residents who were living in the building.
The American Red Cross responded and is assisting the displaced residents, at least six of whom will require long-term housing, according to fire officials.
No one was injured in the fire. The blaze does not appear to be suspicious but its cause has yet to be determined, Keefe said.