The blaze was reported at Petaluma Poultry Processors, located at 2700 Lakeville Highway, at 3:10 p.m.
Responding crews saw heavy smoke and flames coming from the roof in the rear of the roughly 40,000-square foot building.
The fire was centered around electrical equipment on the roof and extended into attic space, fire officials said.
All of the plant's employees were evacuated and no injuries were reported.
PG&E crews and building maintenance workers shut off power to the building and firefighters extinguished the blaze. Fire crews finished an extensive overhaul by 6 p.m., fire officials said.
Public health authorities were notified of a fire in a food processing plant and a Food and Drug Administration inspector responded to the site.