Symmetry-protected resonances can be made to couple with free space by introducing a small degree of geometric asymmetry, leading to controllably sharp spectral response. Here, we experimentally demonstrate a broken-symmetry metasurface for the technologically important low millimeter wave spectrum. The proposed metasurface is fabricated on an ultrathin polyimide substrate, resulting in a low loss and flexible structure. Measurements inside an anechoic chamber experimentally verify the theoretically predicted sharp spectral features corresponding to quality factors of several hundreds. The demonstrated sharp response is also observed with the complementary structure, which responds to the orthogonal linear polarization (Babinet's principle). The designed metasurfaces can be exploited in diverse applications favored by a controllably sharp spectral response, e.g., filtering, sensing, switching, and …