The present invention relates to a fuel tank mounting structure for a vehicle which includes a rear floor panel forming a rear portion floor, a first cross member disposed in front of and under the rear floor panel and extending in a vehicle width direction, and a second cross member disposed rearward of the first cross member and extending in the vehicle width direction. A fuel tank is supported by belts stretched between the first and the second cross members so as to abut on an undersurface of the rear floor panel via a resilient pad. Brackets are joined to the first cross member such that they project rearward of the first cross member, each of the belts is connected to the first cross member by the corresponding bracket, and the fuel tank abuts on a rear surface of each bracket via a second resilient pad.