No strict weight limit (must fit in carrier under seat)
Airline dog policy
Southwest Airlines
Southwest Airlines is a popular US low-cost carrier that allows small dogs in the cabin on all domestic US flights. The airline does not offer any cargo or checked baggage pet service — cabin is the only option. Southwest's major advantage is its extensive domestic US network with no change fees, making it flexible for pet owners who may need to adjust travel dates. The $125 each-way fee is competitive and consistent, with no premium for different routes.
Book early and confirm aircraft acceptance.
Under-seat dimensions can vary by aircraft.
Fees can change by region and currency.
Booking tip
Southwest cannot prebook pets — you must notify the airline at check-in and availability is first-come, first-served with a limit of 6 pets per flight. Arrive early to secure your spot. Only one carrier per passenger. The carrier must be leak-proof and ventilated, fitting completely under the seat. Southwest only allows one pet per carrier. Unlike most US carriers, Southwest does not fly internationally (except limited Caribbean routes), so this is a domestic-only option.
Why it can work
Southwest's no-change-fee policy is a genuine advantage for pet travel. If your dog's health certificate timing shifts or a vet appointment moves, you can adjust your flight without penalty. The airline's point-to-point network means fewer connections and shorter travel times for many domestic US routes. Southwest also does not charge for the first two checked bags, which helps when travelling with extra pet supplies. The consistent $125 fee regardless of route makes budgeting simple.
Restrictions
Domestic US routes only (plus limited Caribbean/Mexico service). No cargo, no checked baggage — cabin only. Maximum 6 pets per flight on a first-come basis. One pet per carrier, one carrier per customer. Pets must remain in the carrier for the entire flight. Not available on flights to/from Hawaii. Dogs must be at least 8 weeks old and fully weaned.
Dog-owner notes
Southwest does not participate in interline agreements, so you cannot connect your pet to another airline's flight. This makes Southwest best suited for point-to-point domestic US travel. The airline does not accept brachycephalic breeds in the cabin. For international travel from the US, you will need a different carrier. Southwest customer service: 1-800-435-9792.
Final check before paying
Confirm the exact aircraft, route, dog breed, carrier size, temperature rules and destination import law directly with the airline. A country can allow dogs while a specific airline or aircraft refuses them.