The California Battalion was formed during the Mexican–American War (1846–1848) in present-day California, United States. It was led by U.S. Army Brevet Lieutenant Colonel John C. Frémont and composed of his cartographers, scouts and hunters and the California Volunteer Militia formed after the Bear Flag Revolt. The battalion's formation was officially authorized by Commodore Robert F. Stockton, commanding officer of the U.S. Navy Pacific Squadron.

The California Republic, or Bear Flag Republic, was an unrecognized breakaway state from Mexico, that for 25 days in 1846 militarily controlled an area north of San Francisco, in and around what is now Sonoma County in California.