Washington Dulles International Airport ( DUL-iss) (IATA: IAD, ICAO: KIAD, FAA LID: IAD) is an international airport in Loudoun County and Fairfax County in Northern Virginia, United States, 26 miles (42 km) west of downtown Washington, D.C.

The airport, which opened in 1962, is named after John Foster Dulles, an influential United States Secretary of State during the Cold War who briefly represented New York in the United States Senate. The airport's main terminal is a well-known landmark designed by Eero Saarinen, who also designed the TWA Flight Center at John F. Kennedy International Airport. Operated by the Metropolitan Washington Airports Authority, Dulles occupies 13,000 acres (20.3 sq mi; 52.6 km2), straddling the Loudoun–Fairfax line. IAD ranks fifth in the US in terms of land area, after Denver International Airport, Dallas/Fort Worth International Airport, Southwest Florida International Airport, and Orlando International Airport. Most of the airport is in the unincorporated community of Dulles in Loudoun County, with a small portion in the unincorporated community of Chantilly in Fairfax County.

Along with Ronald Reagan Washington National Airport (DCA) and Baltimore/Washington International Airport (BWI), Dulles is one of three major airports serving the Washington–Baltimore metropolitan area. As of 2021, it is the second-busiest airport in the Washington–Baltimore metropolitan area behind Reagan National Airport and the 28th-busiest airport in the United States. Dulles has the most international passenger traffic of any airport in the Mid-Atlantic outside the New York metropolitan area, including approximately 90% of the international passenger traffic in the Baltimore–Washington region. It had more than 20 million passenger enplanements every year from 2004 to 2019, with 24 million enplanements in 2019. An average of 60,000 passengers pass through Dulles daily to and from more than 139 destinations around the world.

Increased domestic travel from Reagan National Airport has eroded some of Dulles's domestic routes.