Recent developments in the wireless communication industry continue to drive the requirements for small, compatible, and affordable reconfigurable antennas. Polarization- reconfigurable antennas can mitigate multipath fading loss and provide a double-transmission channel for frequency reuse. A polarization-reconfigurable slot antenna, for wireless communica- tion, is presented in this paper. In the proposed design a new shape of slot antenna based on SIW structures at the center frequency 31 GHz is presented which is linear-polarization (LP)/circular- polarization (CP) switchable reconfigurable antenna while the antenna is still operable over the desired frequency band. The proposed antenna is designed and simulated using Ansoft HFSS, a Finite-Element-Method based EM solver. The computed return loss, for each case, is better than 10 dB over the 30–32 GHz frequency range. The 3 dB axial ratio BW around is 11% in the CP state.