Abstract We have investigated numerically and experimentally an ultra-wideband efficient polarization conversion of a linearly polarized terahertz wave in a planar metamaterial (MM) geometry comprising an array of C-shaped resonators. Both the co- and cross-polarization transmission spectra of the proposed MM configuration have been studied for numerous polarizations of incident terahertz starting from 0^∘ to 90^∘ in steps of 15^∘. Our design promises a cross-polarization conversion of 40% from 1.22 to 2.75 THz with a maximum of 45% when the MM design is rotated by 45^∘ with respect to the incident light. The change in rotation angle excites an electric dipole in the C resonator, which radiates an electric field in the co-and cross-polarization. The polarization conversion has been explained with induced electric field profiles as well as surface current profiles. It is shown that the change in dimension of the C-shaped resonator causes a shift in the transmission spectrum. Numerically simulated results are in good agreement with the experimental results. Our design eliminates the complexity of the fabrication of multilayer structures and opens avenues to design ultra-wideband and efficient polarization rotation devices in the terahertz frequency regime.