The new flight plans that begin July 1 will include daily non-stop flights between Atlanta and Aruba and Montego Bay, Jamaica, and from Baltimore/Washington International to Aruba, Nassau and twice daily to Montego Bay.
Southwest is also scheduling flights on Saturdays only from Orlando, Fla., to Aruba and from Orlando to Jamaica.
Southwest already operates international routes through its AirTran subsidiary, which it bought in 2011.
AirTran flies routes between Atlanta and Nassau and from Chicago Midway to Montego Bay, as well as routes that include Cancun, Los Cabos and Mexico City in Mexico and Punta Cana, Dominican Republic.
For the second phase of the launch announced Monday, Southwest said it would add the four international destinations to the Southwest schedule.
"Southwest Airlines democratized the sky from our first flights more than four decades ago. Today's milestone enables us to reach new territory, new customers, and build upon a four decade foundation of doing right by the travelers who trust our value and our people," Chief Executive Officer Gary Kelly said.