This paper presents ultrathin, broadband, and highly efficient terahertz metamaterial-based polarization converters capable of rotating linear polarization states into their orthogonal counterparts. The devices also enable near-perfect anomalous refraction. The polarization converters are based on a metal cut-wire array and a metal ground plane separated by a dielectric spacer. The incident wave excites dipolar oscillations in the cut-wires, leading to co- and cross-polarized scattering. The ground plane and cut-wire array form a Fabry-Pérot-like cavity, enabling interference that enhances or reduces the overall reflected fields. The device operates in reflection and transmission modes, with the transmission mode using a metal grating to transmit cross-polarized waves while acting as a ground plane for co-polarized waves. The device achieves broadband and high-performance linear polarization conversion with a conversion efficiency exceeding 50% over a wide frequency range. The results also demonstrate broadband near-perfect anomalous refraction, achieved by eliminating ordinary beams. The devices are ultrathin and operate in the technologically relevant terahertz frequency range, enabling advanced applications such as high-performance spatial light modulators. The work opens new opportunities for creating high-performance photonic devices and enables emergent metamaterial functionalities in the terahertz regime. The results are supported by numerical simulations and experimental measurements, showing excellent agreement between the simulations, experiments, and theoretical predictions. The devices are fabricated using photolithography, electron-beam metal deposition, and lift-off techniques. The results demonstrate the potential of metamaterials for advanced polarization control and wavefront shaping in the terahertz frequency range.This paper presents ultrathin, broadband, and highly efficient terahertz metamaterial-based polarization converters capable of rotating linear polarization states into their orthogonal counterparts. The devices also enable near-perfect anomalous refraction. The polarization converters are based on a metal cut-wire array and a metal ground plane separated by a dielectric spacer. The incident wave excites dipolar oscillations in the cut-wires, leading to co- and cross-polarized scattering. The ground plane and cut-wire array form a Fabry-Pérot-like cavity, enabling interference that enhances or reduces the overall reflected fields. The device operates in reflection and transmission modes, with the transmission mode using a metal grating to transmit cross-polarized waves while acting as a ground plane for co-polarized waves. The device achieves broadband and high-performance linear polarization conversion with a conversion efficiency exceeding 50% over a wide frequency range. The results also demonstrate broadband near-perfect anomalous refraction, achieved by eliminating ordinary beams. The devices are ultrathin and operate in the technologically relevant terahertz frequency range, enabling advanced applications such as high-performance spatial light modulators. The work opens new opportunities for creating high-performance photonic devices and enables emergent metamaterial functionalities in the terahertz regime. The results are supported by numerical simulations and experimental measurements, showing excellent agreement between the simulations, experiments, and theoretical predictions. The devices are fabricated using photolithography, electron-beam metal deposition, and lift-off techniques. The results demonstrate the potential of metamaterials for advanced polarization control and wavefront shaping in the terahertz frequency range.