A support structure for a water-cooled internal combustion engine supported by a body frame of a motorcycle 2 includes a cooling device that circulates cooling water through an inside and an outside of at least a cylinder head of the engine, a thermo-valve that controls the cooling water circulating in the cooling device, and a thermo-valve case that accommodates the thermo-valve and that is integrally molded with the cylinder head such that a part thereof bulges from a side surface of the cylinder head. A support portion formed on the cylinder head is fixed to the body frame and is integrally molded so as to stride between the cylinder head and the thermo-valve case, whereby the support portion is solidly formed and an area around a thermo-valve case can be reinforced by the support portion.