Rechargeable Mg batteries (RMBs) are a promising large-scale energy-storage technology with low cost and high safety, but the performance is limited by the inferior kinetics of Mg-intercalation cathodes. In the present study, an octylamine-supporting interlayer expanded molybdenum diselenide (e-MoSe₂) is synthesized and used as cathode for RMBs, in comparison with ordinary crystalline MoSe₂. The octylamine molecules introduced show a strong interaction with the MoSe₂ layers and increase the layer spacing significantly from 6.46 to 11.5 Å. e-MoSe₂ shows a high Mg-storage capacity of 238 mAh g⁻¹ at 50 mA g⁻¹ and a superior rate performance of 39 mAh g⁻¹ at 10 A g⁻¹, far advantageous over crystalline MoSe₂. e-MoSe₂ also shows a considerably high structure stability during repeated magnesiation/demagnesiation, providing an outstanding cycling stability for 1000 cycles. Further electrochemical tests demonstrate the high Mg²⁺ diffusion coefficients in e-MoSe₂. Theoretical computation indicates the interlayer expansion changes the Mg²⁺ diffusion paths from “hollow site → hollow site” to “hollow site → Se atom site → hollow site”, largely decreasing the energy barrier and improving the Mg²⁺ diffusion kinetics. The present work highlights an efficient strategy for the improvement of Mg-storage performance for RMB cathodes.