We present a high-efficiency crankless reciprocating steam engine that loses only a small amount of energy when the rectilinear motion of its piston is changed into the rotary motion of its driveshaft. The present invention continuously rotates a valve in one direction while alternately introducing steam into two piston housing chambers to generate the rotary force of the driveshaft. Therefore, engine efficiency is greatly increased because the inertial force losses in the valve are much smaller compared to the case when the valve rotation stops or changes direction.