Fire Chief Pat Roberts says crews were called to the scene around 5:30.
When firefighters arrived on scene, they found a transit bus inside the storage building fully engulfed in flames.
Crews got the blaze under control within an hour, avoiding it from spreading to other buses inside the building.
Roberts says the bus sustained heavy damage and is a total loss. The building's interior had minor smoke and flame damage.
The fire was reported by a transit worker. There were working smoke alarms in the building, and Roberts says they did go off.
The tribal transit system connects people to the reservation's three main villages and the casino. Bus service was not disrupted because of the fire.
The cause of the blaze is unknown.