The fire was reported at 10:30 a.m. at a four-story building at 755 Railroad Ave. that once housed an employment office, Vallejo Fire Department Battalion Chief Patrick Dunn said.
The roughly 80,000-square-foot building has been vacant for at least 10 years and has often been occupied by homeless people, Dunn said.
The fire was confined to the first floor and was controlled in 30 minutes, Dunn said. Firefighters left the scene around noon, Dunn said.
The fire started near a window on the first floor and caused an estimated $25,000 in damage, Dunn said.