The pilot discharged a fire extinguisher and firefighters met the plane.
A Southwest spokeswoman, Brandy King, said there was no fire. She said the problem was a malfunction with the sensor in the cargo area.
FAA spokesman Allen Kenitzer said the pilot believed there was a fire in the forward storage area and discharged the halon fire extinguisher. He said once it was discharged, the light went out.
The plane landed without incident at Sea-Tac, its destination.