A compact millimeter-wave (mm-wave) 3-D-printed filtering magic-T (FMT) based on half-mode groove gap waveguide (HM-GGW) is proposed in this letter for the first time. Four horizontally polarized HM-GGW resonators (HM-GGWRs) are elaborately utilized to create a compact second-order FMT topology. Magnetic and electrical couplings in the HM-GGW are exploited to achieve the required in-phase/out-of-phase output characteristics, respectively. For demonstration, a prototype was fabricated by 3-D-printing technology, and the measured results agree well with the simulation results.