The Old Custom House (Spanish: Antigua Aduana) is the oldest surviving government building in California, built in 1827 by Mexican authorities in Monterey, then the capital of Alta California.

To collect customs duties (tax monies) at the Monterey Bay port, the Mexican government built the Custom House, making it the oldest government building in present-day California.

On July 7, 1846, during the Mexican–American War, U.S. Commodore John Drake Sloat raised the American flag, declaring California part of the United States during the American Conquest of California.

The Monterey Custom House was a landmark that the Native Sons of the Golden West determined should not disappear if within their power to prevent it.

The property belonged to the United States Government, but the Native Sons obtained a lease of the buildings and grounds and restored them in the early 1900s.