Seattle-Tacoma International Airport (SEA) opened for scheduled commercial service on September 1, 1947, when Northwest Airlines and Western Airlines launched 10 daily departures from a newly completed passenger terminal. Construction on the site near Bow Lake, midway between Seattle and Tacoma, had begun in 1943, funded by the federal government, the Port of Seattle, and the City of Tacoma. The airport was formally dedicated on July 9, 1949, drawing more than 30,000 people. Before that, early passengers made do with a Quonset hut heated by a potbelly stove.

Today SEA is the busiest airport in the Pacific Northwest and, as of 2025, the 11th-busiest in the United States, having set an all-time passenger record of 52,715,181 that year. It is the primary hub for Alaska Airlines, whose headquarters sit near the airport, and a major international gateway for Delta Air Lines, which began building a transpacific hub there in 2014. The airport sits in the city of SeaTac, a municipality that took its name directly from the airport's nickname, about 14 miles south of downtown Seattle.

The airport's layout centers on a single Central Terminal with four concourses (A, B, C, and D) branching outward in an X-formation, plus 2 detached satellite terminals connected by the SEA Underground automated people-mover. The South Satellite is the primary international arrivals facility, a role formalized when a new international arrivals building opened in 2022. The airport operates 3 parallel north-south runways. The third cost $1.1 billion and required decades of planning against local opposition before opening in November 2008. Its specific spacing, 2,500 feet west of the existing eastern runway, allows simultaneous landings in low-visibility conditions, a critical operational advantage in the frequently overcast Pacific Northwest.