The airline announced the move Monday, saying the change takes effect Aug. 11 at Gerald Ford International Airport.
Southwest says it will operate daily flights from Grand Rapids to Baltimore-Washington, Denver, Orlando and St. Louis international airports.
The airline says it will have six daily flights from Grand Rapids, compared with AirTran's three. It's cutting fares to promote the move for tickets bought by Feb. 17.
Southwest bought AirTran in 2011.
Southwest says it also will absorb AirTran flights from Flint Bishop International Airport to Baltimore-Washington, Orlando and Tampa Bay, and it will add a flight from Flint to Las Vegas on Aug. 11.