Provided is a battery module in-vehicle installation structure, including: a pair of side frames that are fixed to a vehicle and arranged at intervals in a width direction of the vehicle; a case body fastened between the pair of side frames; and a mounting member for mounting the battery module inside the case body. The case body has a floor, side walls standing upright at peripheral edges of the floor, and fastening portions extended respectively from the side walls toward an outer side in the width direction. The mounting member has a pair of fixing portions each extended toward the outer side in the width direction. Each of the pair of fastening portions is fastened so as to be sandwiched between one of the pair of fixing portions and one of the pair of side frames.