In this paper, a switchable slot antenna with four polarization states excited by a single port is proposed. The slot radiator is coplanar with a coplanar waveguide, which is excited by a microstrip line on the other side of the substrate. To demonstrate polarization diversity and achieve impedance matching for each state, the PIN diodes were used as the switching elements to select the particular feeding structure and the associated matching network. Experimental results show that this antenna can exhibit right-handed and left-hand circular polarizations, vertical and horizontal (linear) polarizations with measured gains of 1.89 dBi, 2.51 dBi, 2.08 dBi, and 2.62 dBi, respectively. The 3-dB axial-ratio bandwidths for the right-hand and left-hand circular polarizations are 9% and 14.6%, respectively. Within the operating bands, good return losses are obtained.