Washington Dulles International Airport (/ˈdʌlɪs/ DUL-iss) (IATA: IAD, ICAO: KIAD, FAA LID: IAD) – commonly known by its former name of Dulles International Airport, by its airport code of IAD, or simply as Dulles Airport – is an international airport in the eastern United States, 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. Its 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 km), straddling the Loudoun–Fairfax line. IAD ranks fourth in the US in terms of land area, after Denver International Airport, Dallas/Fort Worth International Airport, and Southwest Florida 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. Dulles overtook Reagan in total enplanements in 2019. In 2018, however, Dulles surpassed Reagan in yearly passenger boardings after having fewer passengers since 2015. Furthermore, it still ranks behind BWI in total annual passenger boardings.

Dulles is a hub for United Airlines and is frequently used by airlines that United has codeshare agreements with, mostly composed of Star Alliance members like Turkish Airlines and Lufthansa.

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.

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.

The main terminal was designed in 1958 by famed Finnish-American architect Eero Saarinen, and it is highly regarded for its graceful beauty, suggestive of flight. The terminal was built without any concourses and gates as all aircraft were parked at remote sites. Passengers were bussed to their aircraft by way of mobile lounges that raised up to the aircraft level, some are still in use today. The first midfield terminal that included gates and jetbridges was constructed in 1985 when New York Air and other airlines began hub operations at Dulles. In the 1990s, the main terminal at Dulles was reconfigured to allow more space between the front of the building and the ticket counters. Additions at both ends of the main terminal more than doubled the structure's length. The original terminal at Taiwan Taoyuan International Airport in Taoyuan, Taiwan, was modeled after the Saarinen terminal at Dulles.

The design included a landscaped man-made lake to collect rainwater, a low-rise hotel, and a row of office buildings along the north side of the main parking lot. The design also included a two-level road in front of the terminal to separate arrival and departure traffic and a federally owned limited access highway connecting the terminal to the Capital Beltway (I-495) about 17 miles (27 km) to the east; the highway system eventually grew to include a parallel toll road to handle commuter traffic and an extension to connect to I-66. The access road had a wide median strip to allow the construction of a passenger rail line, which opened as an extension of the Washington Metro's Silver Line on November 15, 2022.

By 1985 the original design, featuring mobile lounges to meet each plane, was no longer well-suited to Dulles's role as a hub airport. Instead, midfield concourses were constructed to allow passengers to walk between connecting flights without visiting the main terminal. Mobile lounges were still used for international flights and to transport passengers between the midfield concourses and the main terminal; Concourse C/D was the first to be built, followed by Concourse A/B. A tunnel (consisting of a passenger walkway and moving sidewalks) that links the main terminal and Concourse B was opened in 2004. The Metropolitan Washington Airports Authority (MWAA) began a renovation program for the airport including a new security mezzanine with more room for lines.

A new train system, dubbed AeroTrain and developed by Mitsubishi, began in 2010 to transport passengers between the concourses and the main terminal. The system, which uses rubber tires and travels along a fixed underground guideway, is similar to the people mover systems at Singapore Changi Airport, Hartsfield–Jackson Atlanta International Airport, and Denver International Airport. The train is intended to replace the mobile lounges, which many passengers found crowded and inconvenient. The initial phase includes the main terminal station, a permanent Concourse A station, a permanent Concourse B station, a permanent midfield concourse station (with access to the current temporary C concourse via a tunnel with moving walkways), and a maintenance facility. Mobile lounges continue to service Concourse D from both the main terminal and Concourse A. Even after AeroTrain is built out and the replacement Concourses C and D are built, the mobile lounges and plane mates will still continue to be used, to transport international arriving passengers to the International Arrivals Building, as well as transport passengers to aircraft parked on hardstands without direct access to jet bridges. Dulles has stated that the wait time for a train does not exceed four minutes, compared to the average 15-minute wait and travel time for mobile lounges.

Under the development plan, future phases would see the addition of several new midfield concourses and a new south terminal. A fourth runway (parallel to the existing runways 1 and 19 L&R) opened in 2008, and development plans include a fifth runway to parallel the existing runway 12–30. If this runway is built, the current runway will be re-designated as 12L-30R while the new runway will be designated 12R-30L. An expansion of the B concourse, used by many low-cost airlines as well as international arrivals, has been completed, and the building housing Concourses C and D will eventually be knocked down to make room for a more ergonomic building. Because Concourses C and D are temporary concourses, the only way to get to those concourses is via moving walkway from the Concourse C station, which is built in the location of the future gates and Concourse D by mobile lounge from the main terminal.

In the short term, United Airlines has constructed a 20,000 square foot (1,900 m) buildout on Concourse C between gate C18 and the AeroTrain entrance for use as a Polaris Lounge for international passengers. Further expansion plans include a new three-story 550,000 square foot (51,000 m) south concourse building above the AeroTrain station for Concourse C, to replace Concourse A regional gates built in 1999.

Decades-old rules set by Congress that limit the number of takeoffs and landings, as well as distance of routes, at Reagan Airport were intended in part to keep more flights at Dulles. Those rules have been weakened by Congress over the years, however, causing Dulles to lose 200,000 passengers to Reagan between 2011 and 2013.

In 2023, construction started on a 100 MW solar power facility, battery and bus charging equipment. It would include the largest airport-based solar and battery development in the U.S. as part of an agreement with Dominion Energy. The solar panels would cover more than 835 acres (338 ha) on land, equivalent to the consumption of more than 37,000 Northern Virginia homes during peak production.

The first scheduled flight at Dulles was an Eastern Air Lines Lockheed L-188 Super Electra from Newark International Airport in New Jersey on November 19, 1962. The first scheduled transatlantic nonstop flight to serve the airport commenced just shy of two years later, in June 1964.

Dulles was initially considered a white elephant, being far out of town with few flights; in 1965, Dulles averaged 89 airline operations a day while National Airport (now Reagan) averaged 600 despite not allowing jets. Airport operations grew along with Virginia suburbs and the Dulles Technology Corridor; perimeter and slot restrictions at National forced long-distance flights to use Dulles. In 1969, Dulles had 2.01 million passengers while National had 9.9 million.

The era of wide-body jets began on January 15, 1970, when First Lady Pat Nixon christened a Pan Am Boeing 747-100 at Dulles in the presence of Pan Am chairman Najeeb Halaby. Rather than a traditional champagne bottle, red, white, and blue water was sprayed on the aircraft. Pan Am's first Boeing 747 flight was from New York–JFK to London Heathrow Airport.

On December 26, 1973, President Richard Nixon flew from Dulles to Los Angeles on board a United Airlines McDonnell Douglas DC-10 commercial flight instead of on Air Force One. This was due to a nationwide fuel shortage caused at the time by the Arab oil embargo. Less than two years later, on May 24, 1976, supersonic flights between the U.S. and Europe began with the arrival of a British Airways Concorde from London–Heathrow and an Air France Concorde from Paris–Charles De Gaulle; both planes were lined nose-to-nose at Dulles for photos.

On June 12, 1983, the Space Shuttle Enterprise arrived at Dulles atop a modified Boeing 747 after touring Europe and before returning to Edwards Air Force Base. Two years later Enterprise returned and was placed in a storage hangar near Runway 12/30 to await construction of a planned expansion to the National Air and Space Museum. Enterprise left Dulles on April 27, 2012, for its new home at the Intrepid Sea, Air & Space Museum in New York City.

On June 1, 1985, New York Air began a small hub operation at Dulles, with 35 daily flights to eight cities in Florida and the northeastern United States. Colgan Airways became a feeder carrier for New York Air with additional service to smaller cities known as New York Air Connection. In 1987, the airline merged into Continental Airlines, which maintained the hub operation until 1989. On October 10, 1985, Presidential Airways opened its hub at the airport, and it soon began a series of code-shares – first with Pan Am from mid-1986 through early 1988, then as Continental Express on behalf of Continental Airlines between mid-1987 and mid-1988, and finally as United Express, on behalf of United Airlines, from mid 1988 until Presidential ceased operations on December 5, 1989.

On May 1, 1986, United Airlines began service on 16 new domestic routes creating a hub status at Dulles. Many more domestic routes and new overseas routes would later be added. Air Wisconsin and Presidential Airways soon became feeder carriers for United operating as United Express.

In 1990, a United States Senate joint resolution to change Dulles International Airport's name to Washington Eisenhower International Airport was proposed by Senator Bob Dole, but the bill didn't pass.

When the SR-71 was retired by the military in 1990, one was flown from its birthplace at United States Air Force Plant 42 in Palmdale, California, to Dulles, setting a coast-to-coast speed record at an average 2,124 mph (3,418 km/h); the trip took 64 minutes. The aircraft was placed in a storage building, and is now displayed at the Smithsonian's adjacent Udvar-Hazy Air and Space Museum.

In 1995, the first flight of the Boeing 777-200 in commercial service landed at Dulles; the flight was operated by United Airlines on its transatlantic London Heathrow – Washington Dulles route.

The 2004 launch of low-cost carrier Independence Air propelled IAD from being the 24th-busiest airport in the United States to fourth, and one of the top 30 busiest in the world. Independence Air ceased operations in January 2006, and its space in Concourse A was taken five months later by United Express. Also taking place in 2006 was the introduction of service by Southwest Airlines at IAD.

Significant growth required the airport to halt the operations of its original control tower in 2007 for a taller control tower located away from the main terminal. The original tower still exists, though it is no longer used to control the airport's traffic. That year, 24.7 million passengers passed through the airport.