Sonoma County (/səˈnoʊmə/ ^ⓘ sə-NOH-mə) is a county located in the U.S. state of California. As of the 2020 United States census, its population was 488,863. Its seat of government and largest city is Santa Rosa.

Sonoma County comprises the Santa Rosa-Petaluma metropolitan statistical area, which is part of the San Jose-San Francisco-Oakland, CA combined statistical area. It is the northernmost county in the nine-county San Francisco Bay Area region.

In California's Wine Country region, which also includes Napa, Mendocino, and Lake counties, Sonoma County is the largest producer. It has nineteen approved American Viticultural Areas and more than 350 wineries. The voters have twice approved open space initiatives that have provided funding for public acquisition of natural areas, preserving forested areas, coastal habitat, and other open space. More than 8.4 million tourists visit each year, spending more than $1 billion in 2016.

Sonoma County is a leading producer of hops, grapes, prunes, and apples, as well as dairy and poultry products, largely due to the extent of available, fertile agricultural land in addition to the abundance of high-quality water for irrigation. Agriculture is largely divided between two nearly monocultural uses: grapes and pasturage.

Scientific studies of rock formations at Wilson Grove in Sonoma County had begun by 1952, and eventually by 1988 provided evidence that around 4 million years ago, there was a shallow sea covering Sonoma County, which was named as the Wilson Grove Sea, distinct from the Merced Sea covering San Francisco during that time. Wilson Grove (named after Walter Wilson) is located near California's Russian River, west of Windsor, California, and from the 1920s onwards has been used for camps and outings. By 1922, fossils had been discovered and analyzed in Wilson Grove, which up to the 1970s was considered as part of the Merced Formation. Sonoma County was identified in 1983 as belonging to the Wilson Grove Formation, deposited from the late Miocene to the late Pliocene; many fossils (including of baleen whales, walruses, sharks, birds, snails, lamp shells, clams, and barnacles) were discovered in 1973 at a quarry at Bloomfield in Sonoma County.

The Pomo, Coast Miwok and Wappo peoples were the earliest human settlers of Sonoma County, between 8000 and 5000 BC, effectively living within the natural carrying capacity of the land. Archaeological evidence of these First people includes a number of occurrences of rock carvings, especially in southern Sonoma County; these carvings often take the form of pecked curvilinear nucleated design.

Spaniards, Russians, and other Europeans claimed and settled in the county from the late 16th to mid-19th century, seeking timber, fur, and farmland. The Russians were the first newcomers to establish a permanent foothold in Sonoma County, with the Russian-American Company establishing Fort Ross on the Sonoma Coast in 1812. This settlement and its outlying Russian settlements came to include a population of several hundred Russian and Aleut settlers and a stockaded fort with artillery. However, the Russians abandoned it in 1841 and sold the fort to John Sutter, settler and Mexican land grantee of Sacramento.

The Mission San Francisco Solano, founded in 1823 as the last and northernmost of 21 California missions, is in the present City of Sonoma, at the northern end of El Camino Real. El Presidio de Sonoma, or Sonoma Barracks (part of Spain's Fourth Military District), was established in 1836 by Comandante General Mariano Guadalupe Vallejo. His duties included keeping an eye on the Russian traders at Fort Ross, secularizing the Mission, maintaining cooperation with the Native Americans of the entire region, and doling out the lands for large estates and ranches. The City of Sonoma was the site of the Bear Flag Revolt in 1846.