Santa Rosa (Spanish for "Saint Rose") is a city in the county seat of Sonoma County, in the North Bay region of the Bay Area in California. Its population as of the 2020 census was 178,127. It is the largest city in California's Wine Country and Redwood Coast. It is the fifth most populous city in the Bay Area after San Jose, San Francisco, Oakland, and Fremont; and the 27th-most populous city in California.

Before the arrival of Europeans, what became known as the Santa Rosa Plain was home to a strong and populous tribe of Pomo people known as the Bitakomtara. The Bitakomtara controlled the area closely, barring passage to others until permission was arranged. Those who entered without permission were subject to harsh penalties. The tribe gathered at ceremonial times on Santa Rosa Creek near present-day Spring Lake Regional Park.

Following the arrival of Europeans, initially Spanish explorers and colonists, the Pomos were decimated by violence, land grab, slavery, genocide and smallpox brought from Europe. Social displacement and disruption followed. There are descendants of survivors still living in the region today.

Santa Rosa was founded in 1833 and named by Mexican colonists after Saint Rose of Lima. The first known permanent European settlement here was the homestead of the Carrillo family of California, in-laws to Mariano Guadalupe Vallejo, who settled the Sonoma pueblo and Petaluma area. In the 1830s, during the Mexican period, the family of María López de Carrillo built an adobe house on their Rancho Cabeza de Santa Rosa land grant, just east of what later became downtown Santa Rosa. By the 1820s, before the Carrillos built their adobe in the 1830s, Spanish and Mexican settlers from nearby Sonoma and other settlements to the south were known to raise livestock in the area.

They slaughtered animals at the fork of the Santa Rosa Creek and Matanzas Creek, near the intersection of modern-day Santa Rosa and Sonoma avenues. This is thought to have been the origin of the name of Matanzas Creek; because it was a slaughtering place, the confluence came to be called La Matanza.

By the 1850s, after the United States annexed California following its victory in the Mexican-American War, a Wells Fargo post and general store were established in what is now downtown Santa Rosa. In the mid-1850s, several prominent locals, including Julio Carrillo, son of Maria Carrillo, laid out the grid street pattern for Santa Rosa with a public square in the center. This pattern has been largely maintained in downtown to this day, despite changes to the central square, now called Old Courthouse Square.

In 1867, the county recognized Santa Rosa as an incorporated city; in 1868, the state officially confirmed the incorporation, making it the third incorporated city in Sonoma County after Petaluma, incorporated in 1858, and Healdsburg, incorporated in 1867.

United States Census Bureau records show that after California became a state, Santa Rosa grew steadily, though it lagged behind nearby Petaluma in the 1850s and early 1860s. In the 1870 census, Santa Rosa was the eighth-largest city in California, and county seat of one of the most populous counties in the state. Growth and development after that was steady but never rapid. The city continued to grow when other early population centers declined or stagnated, but by 1900 it was being overtaken by many other newer population centers in the San Francisco Bay Area and Southern California.