On-demand polarization control of electromagnetic waves is the fundamental element of modern optics. Its interest has recently been expanded in the terahertz (THz) range for coherent excitation of collective quasiparticles in matters, triggering a wide variety of non-trivial intriguing physics, e.g., anharmonicity, nonlinear coupling, and metastability. Wavelength tunability in THz polarization control is fundamentally important for the resonant excitation of collective modes. Here, we propose and demonstrate a simple and convenient THz phase retarder based on the Mach–Zehnder interferometer to obtain circular polarization. The efficiency of THz polarization conversion is demonstrated by the achieved high polarization degree of more than 99.9% and a large transmission of ~ 76%. The simple setup allows us to adapt the phase retarder to existing setups readily and will contribute to further exploration of ultrafast science, e.g., chiral phononics.