A vehicle body mounting structure for an exhaust system heat exchanger is provided that enables good protection of an exhaust system heat exchanger disposed below a vehicle body floor. In the vehicle exhaust system mounting structure 10 there is an exhaust system heat exchanger 14, carrying out heat exchange between exhaust gas and engine cooling water, disposed at the bottom side of a floor tunnel 70 that is formed to a front floor panel 68. The lowermost portion of the exhaust system heat exchanger 14 in the vehicle up-down direction is positioned in the vehicle up-down direction above the lowermost portion of the body cross-member 80 of a vehicle body frame and an engine rear mount support member 90.