The California Republic (Spanish: La República de California), 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.In June 1846, thirty-three American immigrants in Alta California who had entered without official permission rebelled against the Mexican department's government. Among their grievances were that they had not been allowed to buy or rent land and had been threatened with expulsion. Mexican officials had been concerned about a coming war with the United States and the growing influx of Americans into California. The rebellion was covertly encouraged by U.S. Army Brevet Captain John C. Frémont, and added to the troubles of the recent outbreak of the Mexican–American War.

The name "California Republic" appeared only on the flag the insurgents raised in Sonoma. It indicated their aspiration of forming a republican government under their control. The rebels elected military officers but no civil structure was ever established. Their flag, featuring a silhouette of a California grizzly bear, became known as the Bear Flag and was later the basis for the official state flag of California.

Three weeks later, on July 5, 1846, the Republic's military of 100 to 200 men was subsumed into the California Battalion commanded by Brevet Captain John C. Frémont. The Bear Flag Revolt and whatever remained of the "California Republic" ceased to exist on July 9 when U.S. Navy Lieutenant Joseph Revere raised the United States flag in front of the Sonoma Barracks and sent a second flag to be raised at Sutter's Fort.