In this paper, we present a new functional MEMS device monolithically integrated in a CMOS technology which is capable to mechanically perform the function of a balun, i.e. convert electrical signals that are unbalanced (single ended) to balanced ones. The RF–MEMS device consists on a 3rd order lateral mode free–free beam with a 48 MHz resonance frequency. The resonator is electrically actuated and capacitively detected allowing a fully integrated system. Two electrode configurations allow the signal phase shift between the two outputs of the RF–CMOS–MEMS device.