Washington Dulles International Airport (/ˈdʌlɪs/ DUL-iss) (IATA: IAD, ICAO: KIAD, FAA LID: IAD) – commonly known simply as Dulles Airport – is an international airport serving the United States's capital city, Washington, D.C. and its surrounding area. It is located 26 miles (42 km) west of downtown Washington, D.C., in Loudoun and Fairfax counties in Northern Virginia.

Opened in 1962, the airport is named after John Foster Dulles, an influential secretary of state during the Cold War who briefly represented New York in the United States Senate. Operated by the Metropolitan Washington Airports Authority, Dulles occupies 13,000 acres (20.3 sq mi; 52.6 km²); IAD ranks third in the United States in terms of land area, after Denver International Airport and Dallas Fort Worth International Airport. The "striking" and "iconic" Eero Saarinen-designed Main Terminal received vast acclaim for its innovative architecture when it was completed and has gone on to win numerous awards.

Dulles is the second busiest of three major airports serving the Washington–Baltimore metropolitan area, behind Baltimore/Washington International Airport (BWI) and ahead of Ronald Reagan Washington National Airport (DCA). As of 2024, it is the 24th-busiest airport in the United States, with 27.3 million enplanements. It had more than 20 million passenger enplanements every year from 2004 to 2019. An average of 60,000 passengers pass through Dulles daily to and from more than 139 destinations around the world. 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.

Dulles is a hub for United Airlines, and it is frequently used by Star Alliance members that United has codeshare agreements with. Dulles is also a hub for regional operators Mesa, GoJet, and CommuteAir, which operate under the United Express brand.

Before World War II, Hoover Field was the main commercial airport serving Washington, on the site now occupied by the Pentagon and its parking lots. It was replaced by Washington National Airport in 1941, a short distance southeast. After the war, in 1948, the Civil Aeronautics Administration began to consider sites for a second major airport to serve the nation's capital. Congress passed the Washington Airport Act in 1950 to provide funding for a new airport in the region. The initial CAA proposal in 1951 called for the airport to be built in Fairfax County near what is now Burke Lake Park, but protests from residents, as well as the rapid expansion of Washington's suburbs during the time, led to reconsideration of this plan. One competing plan called for the airport to be built in the Pender area of Fairfax County, while another called for the conversion of Andrews Air Force Base in Prince George's County, Maryland, into a commercial airport.

The current site was selected by President Eisenhower in 1958; the Dulles name was chosen by Eisenhower's aviation advisor Pete Quesada, who later served as the first head of the Federal Aviation Administration. As a result of the site selection, the unincorporated, largely African-American community of Willard, which once stood in the airport's current footprint, was demolished, and 87 property owners had their holdings condemned.

Dulles was also built over a lesser-known airport named Blue Ridge Airport, chartered in 1938 by the U.S. The airport was Loudoun County's first official airport, consisting of two grass intersecting runways in the shape of an "X". The location of the former Blue Ridge Airport sits where the Dulles Air Freight complex and Washington Dulles Airport Marriott now sit today.^{[better source needed]}

The civil engineering firm Ammann and Whitney was named lead contractor. The airport was dedicated by President John F. Kennedy and Eisenhower on November 17, 1962. As originally opened, the airport had three long runways (current day runways 1C/19C, 1R/19L, and 12/30) and one shorter one (where current taxiway Q is located). Its original name, Dulles International Airport, was changed in 1984 to Washington Dulles International Airport.