A compact relativistic magnetron (RM) system with diffraction output is proposed, whose novelty lies in the omnidirectional radiation through the slotted waveguide array antenna. In the array antenna, a periodically slotted ridged circular waveguide is employed to obtain omnidirectional radiation and decrease its radius. Meanwhile, the RF field is directly coupling and radiating without complex mode conversion. Moreover, the interaction resonant system applies a ten-cavity rising-sun slow-wave structure to optimize the dispersion characteristics of the operating mode. The particle-in-cell simulations demonstrate that the RM could achieve 70% device efficiency and gigawatt radiation with an operating voltage of 400 kV. Besides, the output RF field is omnidirectional radiating with a gain up to 12.77 dBi at the operating frequency of 1.32 GHz, which is likely to be used in the high-power microwave area defense.