The City and County of San Francisco is the fourth most populous city in California and the 13th most populous city in the United States, with a 2007 estimated population of 799,183. One of the most densely populated major American cities, it is the financial, cultural, and transportation center of the larger San Francisco Bay Area, a region of more than 7 million people. The city is located at the northern end of the San Francisco Peninsula, with the Pacific Ocean to the west and San Francisco Bay to the east.

In 1776, the Spanish established a fort at the Golden Gate and a mission named for Francis of Assisi. The California Gold Rush in 1848 propelled the city into a period of rapid growth, transforming it into the largest city on the West Coast at the time. After being devastated by the 1906 earthquake and fire, San Francisco was quickly rebuilt, hosting the Panama-Pacific International Exposition nine years later. During World War II, San Francisco was the send-off point for many soldiers to the Pacific Theater. After the war, the confluence of returning servicemen, massive immigration, liberalizing attitudes, and other factors gave rise to the Summer of Love and the gay rights movement, cementing San Francisco as a liberal bastion in the United States.

San Francisco is a popular international tourist destination renowned for its chilly summer fog, steep rolling hills, eclectic mix of Victorian and modern architecture and its famous landmarks, including the Golden Gate Bridge, the cable cars, and Chinatown. The city is also known for its diverse, cosmopolitan population, including large and long-established Asian American and LGBT communities.

San Francisco is located on the West Coast of the U.S. at the tip of the San Francisco Peninsula and includes significant stretches of the Pacific Ocean and San Francisco Bay within its boundaries. Several islands—Alcatraz, Treasure Island, and the adjacent Yerba Buena Island, plus small portions of Alameda Island, Angel Island, and Red Rock Island—are part of the city. Also included are the uninhabited Farallon Islands, 27 miles (43 km) offshore in the Pacific Ocean. The mainland within the city limits roughly forms a "seven-by-seven-mile square," a common local colloquialism referring to the city's shape, though its total area, including water, is nearly 232 square miles (600 km2).
Cars negotiate Lombard Street to descend Russian Hill.

San Francisco is famous for its hills. There are more than 50 hills within city limits. Some neighborhoods are named after the hill on which they are situated, including Nob Hill, Pacific Heights, and Russian Hill. Near the geographic center of the city, southwest of the downtown area, are a series of less densely populated hills. Twin Peaks, a pair of hills resting at one of the city's highest points, forms a popular overlook spot. San Francisco's tallest hill, Mount Davidson, is 925 feet (282 m) high and is capped with a 103 foot (31 m) tall cross built in 1934. Dominating this area is Sutro Tower, a large red and white radio and television transmission tower.

The San Andreas and Hayward Faults are responsible for much earthquake activity, even though neither passes through the city itself. It was the San Andreas Fault which slipped and caused the earthquakes in 1906 and 1989. Minor earthquakes occur on a regular basis. The threat of major earthquakes plays a large role in the city's infrastructure development. The city has repeatedly upgraded its building codes, requiring retrofits for older buildings and higher engineering standards for new construction. However, there are still thousands of smaller buildings that remain vulnerable to quake damage.

San Francisco's shoreline has grown beyond its natural limits. Entire neighborhoods such as the Marina and Hunters Point, as well as large sections of the Embarcadero, sit on areas of landfill. Treasure Island was constructed from material dredged from the bay as well as material resulting from tunneling through Yerba Buena Island during the construction of the Bay Bridge. Such land tends to be unstable during earthquakes; the resultant liquefaction causes extensive damage to property built upon it, as was evidenced in the Marina district during the 1989 Loma Prieta Earthquake.

San Francisco has a Mediterranean climate with dry summers, mild year-round temperatures and plenty of sunshine. Temperatures vary little throughout the year. The dry period of May to October is mild to warm, with average high temperatures of 64-70°F (17-21°C) and lows of 51-56°F (10-13°C). The rainy period of November to April is cool with high temperatures of 56-64°F (13-17°C) and lows of 46-51°F (7-10°C). On average, temperatures exceed 75°F (24°C) 28 days a year.

The combination of cold ocean water and the high heat of the California mainland create the city's characteristic fog that can cover the western half of the city all day during the spring and early summer. In fact, a quotation incorrectly attributed to Mark Twain is "The coldest winter I ever spent was a summer in San Francisco." The fog is less pronounced in eastern neighborhoods, in the late summer, and during the fall, which are the warmest months of the year. Due to its sharp topography and maritime influences, San Francisco exhibits a multitude of distinct microclimates. The high hills in the geographic center of the city are responsible for a 20 percent variance in annual rainfall between different parts of the city. They also protect neighborhoods directly to their east from the foggy and cool conditions experienced in the Sunset District; for those who live on the eastern side of the city, San Francisco is sunnier, with an average of 260 clear days, and only 105 cloudy days per year.

Annual precipitation is about 20.4 inches (510 mm) which occurs mainly during the cooler months of November through April. On average, there are 67 rainy days a year.

Ocean Beach runs along the Pacific Ocean shoreline, but is not suitable for swimming because the waters off the coast are perennially cold and form deadly rip currents. Baker Beach is located in a cove just inside the Golden Gate and adjacent to the Presidio, a former military base. Crissy Field, within the Presidio, has been restored to its natural salt marsh ecosystem. All of these together, plus other sites such as Alcatraz and Fort Funston, form part of the Golden Gate National Recreation Area, a regional collection of beaches, parks, and historic sites administered by the National Park Service. The NPS separately administers the San Francisco Maritime National Historic Park—a fleet of historic ships and waterfront property around Aquatic Park.

There are more than 200 parks maintained by the San Francisco Recreation and Parks Department. The largest and best-known city park is Golden Gate Park, which stretches from the center of the city west to the Pacific Ocean. Once covered in native grasses and sand dunes, the park was conceived in the 1860s and was created by the extensive planting of thousands of non-native trees and plants. The large park is rich with cultural and natural attractions such as the Conservatory of Flowers, Japanese Tea Garden and San Francisco Botanical Garden. Lake Merced is a fresh-water lake surrounded by parkland and near the San Francisco Zoo, a city-owned park which houses more than 250 animal species, many of which are designated as endangered. The only park managed by the California State Park system located principally in San Francisco, Candlestick Point was the state's first urban recreation area.