A millimeter-wave antenna is presented that has an adjustable polarization. The polarization can be controlled by the input signal of the antenna and therefore no separate RF switch is needed. A completely planar feed network has been designed that employs branch-line couplers that allow to vary polarization depending on the input signal. The antenna has been realized and the concept of polarization diversity is demonstrated. Measurement results are in agreement with simulated results and validate the performance of the antenna.