Three airports serve Seattle — Sea-Tac (SEA) is the main international gateway with Link Light Rail direct to Stadium Station; Paine Field (PAE) in Everett offers a quieter regional option; Bellingham (BLI) serves the far north. Most World Cup visitors will fly into SEA.
Seattle-Tacoma International (SEA) is the only major international airport serving Seattle — 13 mi south of downtown, ranked among the busiest US airports. SEA is one of the very few US World Cup venues with direct light rail to the stadium: the Link 1 Line connects SeaTac/Airport Station to Stadium Station in 43 minutes for $3.50.
The Satellite Transit System (STS) underground train connects the Main Terminal with the North and South Satellites — runs continuously, free. Allow ~10 min between concourses on the STS or longer on foot.
If domestic fares are significantly cheaper into PAE or BLI, here's what to expect. Both add travel time vs. SEA.