San Francisco International Airport (IATA: SFO, ICAO: KSFO, FAA LID: SFO) is an international airport in an unincorporated area of San Mateo County, 13 miles (21 km) south of Downtown San Francisco. It has flights to points throughout North America and is a major gateway to Europe, the Middle East, Asia, and Oceania.

SFO is the largest airport in the San Francisco Bay Area and the second-busiest in California, after Los Angeles International Airport (LAX). In 2017, it was the seventh-busiest airport in the United States and the 24th-busiest in the world by passenger count. It is the fifth-largest hub for United Airlines, which operates out of Terminal 3 and the International Terminal. SFO functions as United's primary transpacific gateway. Additionally, SFO is a major maintenance hub for United Airlines and houses the SFO Museum that was created in 1980, the first museum in an international airport. It also serves as a hub for Alaska Airlines, which operates in Terminal 2.

The airport is owned and operated by the City and County of San Francisco and lies in a ZIP code assigned to San Francisco. However, it is located outside of San Francisco in unincorporated San Mateo County. Between 1999 and 2004, the San Francisco Airport Commission operated city-owned SFO Enterprises, Inc., to oversee its business purchases and operations of ventures.