Pioneer Building, Commercial landmark in San Francisco, United States.

The Pioneer Building is a three-storey grey-blue corner building at 2185-2199 Folsom and 3180 18th Streets, built in 1902 to Thomas J. Welsh's design. Red capital letters reading "PIONEER BUILDING" cross the facade between the first and second floors, with a red band below the roofline. It began as two adjoining wooden structures along 18th Street, joined into one in 1985-1986.

Built in 1902 to designs by architect Thomas J. Welsh, the Pioneer Building began as two adjoining structures on 18th Street, home to the Pioneer Trunk Factory and a corner storefront. The factory made automobile trunks until the 1930s, after which the building served as a warehouse and machine shop. It was joined into one during a 1985–1986 rehabilitation and listed on the National Register of Historic Places in 1987 as the Pioneer Trunk Factory--C. A. Malm & Co.

The building's Italianate wooden ornament imitates the Victorian houses around it, an unusual choice for a factory. Its name comes from the Pioneer Trunk Factory, which made automobile trunks sold in two downtown San Francisco stores. A corner storefront operated alongside the factory.

The building stands in an urban district where visitors can view the facade from the street level. Those interested should note that it remains an active commercial property with interiors not generally open to the public.

The building is one of the few surviving large all-wood industrial structures in the Mission district, ornamented with Italianate detail; its wooden style was unusual for a factory and may have been chosen to fit the surrounding houses. It sits just outside the area destroyed by the 1906 fire.