The design of a novel reflective linear-cross and linear-circular polarizer at the sub-THz regime is demonstrated in this study. The proposed polarizer's unit cell consists of foursquare patches at corners, and two corners are connected with metallic traces to form an electric dipole. Polyamide substrates are used for printing the FSS, with one side serving as a ground plane. A linearly polarized incident wave is transformed into its orthogonal polarization in three bands during reflection, 0.46- 0.50THz, 1.08-1.57THz, and 2.00-2.02 THz, according to the results of the post-simulation. From 0.43-0.45 THz, 0.53-0.94 THz, 1.61-1.64 THz, 1.96-1.99 THz, and 2.04-2.06 THz, the reflected wave would also be circularly polarized. Having an overall effective thickness of 0.043λL and a periodicity of 0.132 λL, the design is small. For linear-circular conversion, the design shows a consistent response up to 30°, and for linear-cross conversion, up to 40°. The authors believe that the proposed geometry offers various benefits, including simplicity, compactness, and better angular stability. Dual-mode multiband polarization conversions are the fundamental issue in real-time applications such as time-domain spectroscopy (TDS) and beam splitters, where the dual-polarized linear-cross and linear-circular conversions will be used. © 2022 IEEE.