A high-efficiency transmitted polarization converter based on a frequency selective surface (FSS) is proposed in this paper. The FSS-based polarization converter (FSS-PC) is designed based on receiving-via-transmitting (RVT) structure. The receiving and transmitting antenna structures are interconnected by the transmission line, designed in the form of metallized via holes. For any linearly polarized (LP) electromagnetic wave, our proposed FSS-PC has the capability to convert it into another LP electromagnetic wave. This converted wave will have a counterclockwise rotation angle of 2φ relative to the incident wave at 11 GHz. This is achieved by adjusting the relative azimuth φ between the polarization plane of the incident LP wave's electric field and the converter. Meanwhile, the FSS-PC can achieve exceptionally high polarization conversion above -0.30 dB at the central frequency of 11 GHz. Furthermore, as the azimuth of the incident electric field varies, this high-efficiency polarization conversion capability remains stable. The prototype has been fabricated and measured, and the measured results agree well with the simulated ones, thus confirming the effectiveness of the proposed design.