Horizon flight No. 404 had departed from San Jose for Boise, Idaho, but returned after reporting a fire in the right engine or the activation of a fire indication light, Federal Aviation Administration spokesman Ian Gregor said.
The plane, a de Havilland DH8, landed at the San Jose airport without incident at 11:30 a.m., Gregor said.
He said there were 48 passengers and six crew members on board.
No injuries were reported, a fire dispatcher said.