A novel gain enhancement method for a dual-polarized open-ended square waveguide antenna is proposed in this paper. Slots are etched on the sidewalls near the input of the open-ended square waveguide, which couple part of the energy to the periodic structures loaded on the sidewalls and excite the TM 0 modes. The surface wave is excited on the periodic structures. The periodic structures are equivalent to a dielectric rod with a graded dielectric constant. Part of the energy coupled to the periodic structures is radiated during the transmission, which combines with the energy radiated by the open waveguide, forming a plane wave to produce a high-gain end-fire beam in the far field. The antenna is printed using Stereo Lithography Appearance (SLA) 3D printing technology, and then a layer of copper is plated on the outer surface to make the antenna equivalent to an all-metal structure. Compared with the dielectric-loaded open-ended waveguide antennas, 3D-printed all-metal periodic structures reduce dielectric loss and processing costs. The antenna structure is a square waveguide, so an orthogonal-mode transducer (OMT) and circular polarizer can be used to achieve dual linear polarization (LP) and dual circular polarization (CP). The simulated results show that the proposed antenna achieves a gain enhancement of about up to 8dB (Compared with an unloaded open waveguide antenna). The measured results show that a peak gain of 17.8 dBi is realized for LP states. The antenna is very suitable for millimeter-wave high-gain application scenarios, such as point-to-point communication applications.