Pilots operating flight No. 450, which departed from SFO at 2:51 p.m. Saturday, noticed a light indicating that there was an overheat in the wheel well, Southwest spokeswoman Brandy King said.
Per protocol, the plane carrying 86 passengers bound for Chicago Midway Airport was diverted to the nearest airport, which was Oakland International, King said.
Once on the ground, fire crews inspected the 737 Boeing jetliner.
"There was no indication of an overheat or a fire," King said.
The passengers deplaned and will board another flight, King said.
"Everyone is safe, waiting to board the next aircraft," King said at about 4:15 p.m.
Maintenance crews are working to determine what caused the indicator light to illuminate, King said.