The research presents a 3D metal printed X-band (8.2-12.4 GHz) horn antenna with side-lobe reduction, achieved by printing pyramidal periodic structures on the inner faces of the antenna's E-plane. The 3D pyramidal periodic structures result in the suppression of the surface current and hence reduce the side-lobes of the horn antenna. The placement of the periodic structures has been optimized using commercially available electromagnetic simulation software, CST (Computer Simulation Technology), not only for reducing side-lobes, but also for successful 3D metal printing. The beam efficiency of the proposed antenna was measured to be 98.5% at the selected frequency of 12.4 GHz, with an improvement of 8% over the 90.4% simulated beam efficiency of the conventional horn antenna without any periodic structures. The antenna is fabricated using the Direct Metal Laser Sintering (DMLS) technique using Maraging Steel, MS1, on an EOSINT M280 system.