Firefighters responded to the three-alarm blaze at Blue Ocean seafood restaurant on the 1400 block of South Garfield Avenue at about 7:40 a.m. Upon arrival, smoke was seen coming from the roof.
The fire was initially burning in the kitchen area, officials said, but flames quickly spread and tore through the restaurant.
"It spread pretty quickly. It was pretty well involved in the attic and the walls before we got here," said Assistant Chief Ray Mosack of the Alhambra Fire Department.
The fire-damaged roof collapsed, and hours after the fire broke out, firefighters were still putting water on the smoldering ruins.
Three people were inside when flames broke out, but everyone was able to get out safely.
Roads in the area were shut down, and drivers were advised to stay away from the area as the mop-up continued.
The cause of the fire is under investigation.