A novel polarization selective surface (PSS) expands the bandwidth of the gain for electrically large modulated metasurface antennas (MMAs). The PSS transforms all surface waves with the same direction to generate a more uniform phase distribution and improve polarization performance above the MMA to enhance its radiation performance over a wider bandwidth. The fabricated antenna realized 30.5 dBi gain at 131 GHz with a measured 3-dB gain bandwidth of 9%, approaching the theoretical limit. The proposed MMA loaded with a PSS produces high gain and low profile THz antennas operating potentially beyond 300 GHz.