A novel design of the polarizer based on a square waveguide with four posts and four irises for Ku‐band 10.7–12.8 GHz is proposed. The main advantage of the developed polarizer design is the option to tune its polarization characteristics by variation of the posts height. The equivalent circuit of the polarizer has been obtained using scattering and transfer matrices technique. Using this model, the polarization and matching characteristics of the suggested polarizer were optimized simultaneously using the trust region method. This method is reliable and robust since it provides good convergence properties. The novelty of this study lies in the use of a new waveguide polarizer structure based on a combination of irises and posts. Posts enable fine‐tuning of the polarizer performance, while irises promote wide operating frequency band. The following simulated characteristics of the optimal polarizer design were obtained: VSWR for the main modes of horizontal and vertical polarization is less than 1.47, the differential phase shift is within 90° ± 2.6°, the axial is less than 0.46 dB, and the XPD is higher than 32 dB in the operating frequency range. A developed polarizer can be applied in antenna feeds for satellite communication and radar systems.