We report on experimental implementation of a ferroelectric metasurface using an x-cut KTiOPO4 (KTP) crystal for efficient manipulation of terahertz (THz) radiation. Based on the multipolar resonances that are accommodated in KTP micro-blocks in a square array, the metasurface is fabricated by precision diamond-blade dicing. Adjusting the size of the KTP micro-blocks to tailor the relative spectral positions of the anisotropic multipolar resonances, we demonstrate a subwavelength-thin THz polarizer that functions as a transparent film in the y-direction and a magnetic mirror in the z-direction with a transmission contrast of 13 dB near 0.37 THz (820 µm). The ferroelectric-based all-dielectric metasurface will provide a versatile platform to engineer the THz waves in the far field and could potentially be combined with THz generation in the same material.