Abstract: A relativistic magnetron with online switchable rotation direction of a circularly polarized TE₁₁ output mode is proposed. In the device, the same cavity magnetron is adopted as the beam-wave interaction structure, the all-cavity extraction structure is adopted as the output structure, and the Helmholtz coils system is adopted as the magnetic system. In this paper, the output mode components of the device are theoretically analyzed by the mode excitation theory of all-cavity extraction structure, and the performance of the device is investigated by Particle in Cell simulation. Simulation results show that when the applied voltage is 770 kV and the applied magnetic field that has the same direction with the output microwave is 0.2 T, the device that operates at 5π/6 mode can output a right circularly polarized TE₁₁ mode with the mode purity of more than 99%, the operating frequency of 2.35 GHz and the output power of 3.86 GW, corresponding to the power conversion efficiencyof 55.5%. When the direction of the applied magnetic field is reversed, the rotation direction of the right circularly polarized TE₁₁ mode can be online switched to the left in the condition of keeping other performance of the device.