We introduce a new reconfigurable linear and cir-cular polarization antenna, based on a Rectangular Waveguide(RWG) and a novel reconfigurable polarizer. The polarizerconsists of two orthogonal apertures. The apertures transformthe linear polarization of the dominant mode of the RWGantenna to circular polarization. Each of these apertures isloaded by a p-i-n diode to alter its electrical length. This lengthdifference provides the phase rotation needed to obtain circularpolarization. The signal polarization of the antenna can beselected by changing the applied bias voltage of p-i-n diodesas Right-Handed (RH)/Left-Handed (LH) Circular Polarization(CP). We can also achieve linear polarization if both diodesare on or off. The reconfigurable polarizer acts as a band-passfilter since the apertures can be considered as complementaryelectrically small resonators. This self-filtering capability is usefulfor reduction out of band noise in communication system suchas satellite links. The proposed antenna is designed to operateat 2.65 GHz, but the same approach can be applied to otherfrequency bands. The measurement results show a very goodRH/LHCP and they are in a good agreement with simulations.