A piston for an internal combustion engine has a body including an upper combustion wall having an upper combustion surface; cylindrical outer wall with a ring belt region adjacent the upper combustion surface, and a closed annular cooling gallery located in radial alignment with the ring belt region. A cooling medium is contained in the cooling gallery. The cooling gallery has an inner surface including a radially outermost portion extending along the ring belt region. The outermost portion converges from the upper combustion wall toward a longitudinal central axis. During reciprocating motion of the piston, the cooling medium flows and remains in contact with the cooling gallery walls, thereby maximizing the capacity for heat to be transferred from the upper combustion wall to the contained cooling medium and from the cooling medium to the piston body, ring belt region and ultimately to the engine cooling system.