There is provided a battery pack mounting structure for an electric vehicle. A battery pack is mounted in a space behind a rear seat of the electric vehicle. Battery modules are accommodated in the battery back are in a stacked state in a height direction of the electric vehicle. A floor panel behind the rear seat is formed with an opening into which an upper part of the battery pack is inserted. When seen from a side of the electric vehicle, a dimension of one battery module disposed at a position protruding upwards from the opening, is smaller in the longitudinal direction than that of another battery module disposed below the one battery module, and a front end portion of the one battery module is offset towards a rear side of the electric vehicle relative to that of the another battery module.