An internal combustion engine for a small planing boat for preventing supercooling of the internal combustion engine before warm-up while enabling efficient cooling of the internal combustion engine after warm-up. An internal combustion engine for a small planing boat includes a first cooling water path B through which cooling water introduced from a jet propulsion pump flows toward an internal combustion engine main body via an exhaust manifold. A second cooling water path C is provided through which cooling water introduced from the jet propulsion pump flows toward an exhaust pipe via an oil cooler. A bypass cooling water path D is provided for branching a part of cooling water in the second cooling water path C which flows out from the oil cooler, so as to merge into cooling water in the first cooling water path B which flows into the internal combustion engine main body.