Seattle–Tacoma International Airport (IATA: SEA, ICAO: KSEA, FAA LID: SEA) is the primary international airport serving Seattle and its surrounding metropolitan area in the U.S. state of Washington. It is in the city of SeaTac, which was named after the airport's nickname Sea–Tac, approximately 14 miles (23 km) south of downtown Seattle and 18 miles (29 km) north-northeast of downtown Tacoma. The airport is the busiest in the Pacific Northwest region of North America and is owned by the Port of Seattle.

The entire airport covers an area of 2,500 acres (1,000 hectares) and has three parallel runways. It is the primary hub for Alaska Airlines, whose headquarters are near the airport. The airport is also a hub and international gateway for Delta Air Lines, which has expanded at the airport since 2011. As of 2022^[update], 31 airlines operate at Sea–Tac, serving 91 domestic and 28 international destinations in North America, Oceania, Europe, the Middle East, and Asia.

Sea–Tac was developed in the 1940s to replace Boeing Field, which had been converted to military use during World War II. A site near Bow Lake was chosen in 1942 and construction began the following year with funding from the federal government, Port of Seattle, and the City of Tacoma. The first scheduled commercial flights from the airport began in September 1947 and the terminal was dedicated on July 9, 1949. Sea–Tac was expanded in 1961 to accommodate jetliners and added new concourses and satellite terminals by 1973. The main runway was extended several times and twinned in 1970; the third runway opened in 2008 following several decades of planning due to local opposition.^{[citation needed]}

Several major concourse expansions and renovations were initiated in the 2010s to accommodate passenger growth at Sea–Tac, which had become a new hub for Delta Air Lines. A new international arrivals facility opened in 2022 as part of the program. In 2023, Sea–Tac served 50,887,260 passengers, 2% below the all-time record set in 2019.

In 2024, Seattle–Tacoma International Airport set an all-time record with 52,640,716 passengers served, breaking the record set in 2019 with 51.8 million passengers, and 3.45% higher than in 2023.

A shared public airport was proposed by the Port of Seattle and Port of Tacoma in the late 1920s, but deferred plans after the legality of public ports operating such facilities was rejected by the state attorney general in a published opinion. The state legislature authorized municipal corporations such as public ports to establish aviation facilities in 1941. The Port of Seattle accepted a $1 million grant (equivalent to $15.1 million in 2024 dollars) from the Civil Aeronautics Administration in March 1942 to construct a new airport to serve the Seattle area after the U.S. military took control of Boeing Field during World War II. A site on U.S. Route 99 near Bow Lake south of Seattle was chosen at the end of the month ahead of another candidate near Lake Sammamish that was considered too close to the Cascade Range. The City of Tacoma provided $100,000 towards the airport's construction costs as part of a deal for the Bow Lake site, centered around an existing private airfield plagued by heavy fog. Construction of the airport, which was named Seattle–Tacoma in recognition of Tacoma's contribution, began with a groundbreaking ceremony on January 2, 1943.

The project was originally estimated to cost $1.7 million (equivalent to $24 million in 2024 dollars), but the sandy soil conditions drove the final construction price to over $4.2 million (equivalent to $59.3 million in 2024 dollars). The airport's plateau was formed through 6.5 million cubic yards (5,000,000 m³) of excavated earth. A proposal to rename the airport for Boeing president Philip G. Johnson shortly after his death in September 1944 was rejected by Port of Seattle commissioners due to the objections of Tacoma. The first ceremonial landing at Seattle–Tacoma Airport was made on October 31, 1944, by a United Air Lines DC-3 carrying local elected officials and civic leaders. The first commercial flights launched in May 1945 with Northwest Airlines, but use was limited due to the U.S. Army Air Force's need for the airport to stage Boeing B-29 bombers for delivery. Various airlines had irregular flights to the airport, which used a Quonset hut with limited heating as a terminal until a permanent building was financed by a bond issue that voters approved in 1946.

The first scheduled commercial flights began on September 1, 1947, with Northwest Airlines and Western Airlines operating ten daily departures. They were joined by United, Alaska, Trans-Canada, Western, and Pan Am by 1951 as airlines departed from Boeing Field. The terminal at the renamed Seattle–Tacoma International Airport was formally dedicated by Governor Arthur Langlie on July 9, 1949, in front of a crowd of 30,000 spectators. The 71,000-square-foot (6,600 m²) building, designed by architect Herman A. Moldenhour, included a rooftop control tower and glass courting walls in the concourses. The 907-acre (367 ha) airport originally had four runways at 45-degree angles, between 5,000 and 6,100 feet (1,500 and 1,900 m) long, for crosswind operations. The two perpendicular runways were arranged into an "X"-shape that intersected near the longest, north–south runway; an additional runway to the south ran east–west. The terminal building's "inverted V" shape was arranged to match the runway layout. The north–south runway (now Runway 16L/34R) was lengthened to 7,500 ft (2,300 m) in 1950, to 8,500 ft (2,600 m) in 1955, and to 10,200 ft (3,100 m) in 1958 to support commercial jetliners.