Southwest Airlines Co., typically referred to as Southwest, is one of the major airlines of the United States and the world's largest low-cost carrier. It is headquartered in Dallas, Texas and has scheduled service to 121 destinations in the United States and ten additional countries. As of 2018, Southwest carried more domestic passengers than any other United States airline. Southwest is the only low cost carrier that is also a Legacy carrier.

The airline was established on March 15, 1967, by Herb Kelleher as Air Southwest Co. and adopted its current name, Southwest Airlines Co., in 1971, when it began operating as an intrastate airline wholly within the state of Texas, first flying between Dallas, Houston and San Antonio. It began regional interstate service in 1979 and began expanding nationwide in the decades that followed. Southwest currently serves airports in 45 states and multiple Central American destinations.

Southwest's business model is distinct from other US airlines, as it uses a rolling hub and point-to-point network, allows free checked baggage, and does not charge change fees. It uses exclusively Boeing 737 jets in its fleet.

The airline has nearly 60,000 employees and operates about 4,000 departures a day during peak travel season.