The airline recently announced similar changes on flights from BWI Thurgood Marshall Airport to destinations such as Aruba, Montego Bay and Nassau.
Southwest acquired AirTran in May 2011.
Flights from BWI to Cancun will come under Southwest starting Aug. 10, the company said. Flights to Aruba, Montego Bay and Nassau begin July 1.
Officials at BWI hope Southwest — the airport's largest carrier — will expand its portfolio of international flights out of the airport in coming years, and Southwest officials have said they are reviewing options to do so.