There is provided a power unit mounting structure including: a power unit mounted at an upper side in a vehicle vertical direction relative to a pair of side members, the side members being spaced apart in a vehicle width direction at a vehicle front side or a vehicle rear side relative to a passenger compartment; a pair of upper side vehicle body structural members of which at least portions are disposed at outer sides in the vehicle width direction at the upper side in the vehicle vertical direction relative to the side members; and a brace that spans between the pair of upper side vehicle body structural members at the upper side in the vehicle vertical direction relative to the side members, the brace supporting a bottom portion of the power unit at the side thereof that is further from the passenger compartment in the vehicle front-and-rear direction.