The California Republic, commonly known as the Bear Flag Republic, was an unrecognized breakaway state from Mexico, that existed from June 14, 1846 to July 9, 1846. It 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 what 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.

By 1845–46, Alta California had been largely neglected by Mexico for the twenty-five years since Mexican independence. It had evolved into a semi-autonomous region with open discussions among Californios about whether California should remain with Mexico; seek independence; or become annexed to the United Kingdom, France, or the United States. In 1845, the widely hated Manuel Micheltorena, the latest governor to be sent by Mexico, was forcefully ejected by the Californians. His forces were defeated at the Battle of Providencia (also known as the Second Battle of Cahuenga Pass) as a result of the actions of California pioneer John Marsh. This resulted in the return of Californian Pio Pico to the governorship. Pico ruled the region south of San Luis Obispo with his capital in The Town of Our Lady the Queen of Angels of the Porciúncula River, now known as Los Angeles. The area to the north of the pueblo of San Luis Obispo was under the control of Alta California's Commandante José Castro with headquarters near Monterey, the traditional capital and, significantly, the location of the Customhouse. Pico and Castro disliked each other personally and soon began escalating disputes over control of the Customhouse income.

Decrees issued by the central government in Mexico City were often acknowledged and supported with proclamations but ignored in practice. By the end of 1845, when rumors of a military force being sent from Mexico proved to be false, rulings by the other district government were mostly ignored.

The relationship between the United States and Mexico had been deteriorating for some time. The Republic of Texas, which Mexico still considered to be its territory, had been admitted to statehood in 1845. Mexico had earlier threatened war if this happened. James K. Polk was elected President of the United States in 1844, and considered his election a mandate for his expansionist policies.

Mexican law had long allowed grants of land to naturalized Mexican citizens. Obtaining Mexican citizenship was not difficult and many earlier American immigrants had gone through the process and obtained free grants of land. That same year (1845), anticipation of war with the United States and the increasing number of immigrants reportedly coming from the United States resulted in orders from Mexico City denying immigrants from the United States entry into California. The orders also required California's officials not to allow land grants, sales or even rental of land to non-citizen emigrants already in California. All non-citizen immigrants, who had arrived without permission, were threatened with being forced out of California.