Heat transfer is measured downstream of perpendicular injection of an air jet through a single hole into a turbulent mainstream boundary layer. The heat transfer coefficient, calculated from wall temperature measurements with a constant heat flux from the test surface, is determined with injection of both heated and unheated jets. The heat transfer coefficient near the hole is as much as 45 percent larger than the value without injection for a blowing rate (mass flux ratio) of 2.0. Even far downstream, the heat transfer coefficient is 10–15 percent greater than the flat plate value for blowing rates greater than 0.2. The increased value of the heat transfer coefficient near the point of injection is due to the high turbulence levels that arise from interaction between the jet and main flow near the point of injection. Significant variations of the heat transfer coefficient with Reynolds number or wall heat flux are not observed.