One of the two primary passenger terminals of Ontario International Airport (ONT), located in Southern California, is Terminal 4, where Southwest Airlines is based. Dedicated to domestic flights, Southwest Airlines ONT Terminal offers a convenient and user-friendly experience that is customized to fit Southwest's effective service model. Southwest Airlines check-in counters are prominently displayed close to the main entrance of Terminal 4. Passengers can check in and drop off checked bags at self-service kiosks or at manned desks. The Southwest app also offers mobile check-in, which lets you bypass the lineups for a more seamless experience.