San Francisco (; Spanish for "Saint Francis"), officially the City and County of San Francisco, is a commercial, financial, and cultural center of Northern California. The city proper is the fourth most populous in California, with 815,201 residents as of 2021, and covers a land area of 46.9 square miles (121 square kilometers), at the end of the San Francisco Peninsula, making it the second most densely populated large U.S. city after New York City and the fifth most densely populated U.S. county, behind only four of the five New York City boroughs. Among the 91 U.S. cities proper with over 250,000 residents, San Francisco was ranked first by per capita income and sixth by aggregate income as of 2021. Colloquial nicknames for San Francisco include SF, San Fran, The City, Frisco, and Baghdad by the Bay.

San Francisco was founded on June 29, 1776, when Settlers from New Spain established the Presidio of San Francisco at the Golden Gate and Mission San Francisco de Asís a few miles away, both named for Francis of Assisi. The California Gold Rush of 1849 brought rapid growth, transforming an unimportant hamlet into a busy port, making it the largest city on the West Coast at the time; between 1870 and 1900, approximately one quarter of California's population resided in the city proper. In 1856, San Francisco became a consolidated city-county. After three-quarters of the city was destroyed by the 1906 earthquake and fire, it was quickly rebuilt, hosting the Panama-Pacific International Exposition nine years later. In World War II, it was a major port of embarkation for naval service members shipping out to the Pacific Theater. It then became the birthplace of the United Nations in 1945. After the war, the confluence of returning servicemen, significant immigration, liberalizing attitudes, the rise of the beatnik and hippie countercultures, the sexual revolution, the peace movement growing from opposition to United States involvement in the Vietnam War, and other factors led to the Summer of Love and the gay rights movement, cementing San Francisco as a center of liberal activism in the United States.

San Francisco and the surrounding San Francisco Bay Area are a global center of economic activity and the arts and sciences, spurred by leading universities, high-tech, healthcare, finance, insurance, real estate, and professional services sectors. As of 2020, the metropolitan area, with 6.7 million residents, ranked 5th by GDP ($874 billion) and 2nd by GDP per capita ($131,082) across the OECD countries, ahead of global cities like Paris, London, and Singapore. San Francisco anchors the 13th most populous metropolitan statistical area in the United States with 4.6 million residents, and the fourth-largest by aggregate income and economic output, with a GDP of $669 billion in 2021. The wider San Jose–San Francisco–Oakland Combined Statistical Area is the fifth most populous, with 9.5 million residents, and the third-largest by economic output, with a GDP of $1.25 trillion in 2021. In the same year, San Francisco proper had a GDP of $236.4 billion, and a GDP per capita of $289,990. San Francisco was ranked seventh in the world and third in the United States on the Global Financial Centres Index as of March 2022. Despite a continuing exodus of businesses from the city center, significantly accelerated by the COVID-19 pandemic, the Bay Area is still the home to four of the world's ten largest companies by market capitalization, and the city proper still houses the headquarters of numerous companies inside and outside of technology, including Wells Fargo, Salesforce, Uber, Airbnb, Twitter, Levi's, Gap, Dropbox, and Lyft. However, the Hoover Institution in California, in addition to various media organizations, have warned of a uniquely severe long-term doom spiral impending for San Francisco.

One of the top tourist destinations in the United States, San Francisco is known for its steep rolling hills and eclectic mix of architecture across varied neighborhoods, as well as its cool summers, fog, and landmarks, including the Golden Gate Bridge, cable cars, Alcatraz, and Chinatown and Mission districts. The city is home to a number of educational and cultural institutions, such as the University of California, San Francisco, the University of San Francisco, San Francisco State University, the San Francisco Conservatory of Music, the de Young Museum, the San Francisco Museum of Modern Art, the San Francisco Symphony, the San Francisco Ballet, the San Francisco Opera, the SFJAZZ Center, and the California Academy of Sciences. Two professional sports teams, MLB's San Francisco Giants, and the NBA's Golden State Warriors, all play their home games within San Francisco proper. San Francisco's main international airport offers flights to over 125 destinations while a light rail and bus network, in tandem with the BART and Caltrain systems, connects nearly every part of San Francisco with the wider region.