High-gain millimeter-wave antennas based on spoof surface plasmon polaritons (SPPs) are proposed in this article. Since the dispersion of spoof SPP modes supported by metallic grooves is insensitive to their lateral width, we propose two millimeter-wave antennas with wide SPP grooves, namely an SPP tapering rod antenna and an SPP leaky-wave antenna. The widened grooves enlarge the aperture size of the antennas, enhancing the antenna gains. Both antennas are fed by a standard waveguide and are fabricated by 3-D printing with surface metallization. Based on the similar principle of conventional dielectric rod antennas, the proposed SPP rod antenna achieves broadband and high gain. The measured |S 11 | is below −16.5 dB from 50 to 75 GHz, and the antenna gain varies from 16.06 to 19.3 dBi. By employing periodic modulation, the SPP leaky-wave antenna operates from 50 to 70 GHz for |S 11 | below −10 dB. This leaky-wave antenna with 24 modulated periods achieves 20.1-23.9 dBi gain in the operating band with a beam-scanning range from −39° to −3°.