MgMn 2 O 4 with a tetragonal spinel structure shows promise as a positiveelectrode material in magnesium rechargeable batteries (MRBs), which have drawn considerable attention as post lithium-ion batteries. However, the material currently suffers from poor cycle performance. In this study, we attempt to improve the cycle performance of MgMn 2 O 4 via the Zr modification of its particle surface. X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy demonstrate that the surface modification is successfully performed by immersing MgMn 2 O 4 powder into a Zr-containing aqueous solution, followed by heat treatment. However, Zr segregation is observed at high Zr concentration. Furthermore, structural analyses using synchrotron X-rays indicate that the Zr modification has an influence on the bulk structure of the MgMn 2 O 4 powder. The positive-electrode properties of the powders are investigated using discharge/charge cycle tests, which show that Zr modification can drastically improve the cycle performance and coulombic efficiency. These improvements are supposed to be due to suppression of an unexpected reaction by the Zrsurface modification and lower structural distortion after the modification. These findings clearly demonstrate the significant potential of surface modification as a method for obtaining high-performance MRBs.