A spin rectifying diode, which utilizes Rashba spin-orbit coupling, is proposed using semiconducting triple barrier structures. This spin diode makes use of spin-dependent resonant tunneling levels that are formed in the triple barrier structures. We found that, for a certain emitter-collector bias voltage, it is possible to engineer the structure in such a way that a resonant l evel formed within the first quantum well matches that of the second quantum well only for a selected spin state, thus realizing an electronic spin rectifier. The calculated spin polarization of the transmitted current through the device, which is defined as I
↓), is found to be higher than 99.9%.