Magnesium rechargeable batteries have recently attracted attention because of the large theoretical volumetric capacity, dendrite-free deposition/stripping behaviour, and earth-abundant Mg resources. However, the sluggish diffusion of Mg²⁺ in host materials has so far limited the choice of cathodes to Chevrel-phase molybdenum chalcogenide that has Mg-ion storage capacity around 100 mAh g^–1. Here we show that a high-performance magnesium rechargeable battery enabled by a MgCl-ion storage mechanism using two-dimensional TiS₂ with an interlayer spacing expanded up to an unusually large value of 1.86 nm (327% as large as the pristine form) can deliver a high reversible capacity of 270 mAh g^–1 and an exceptional rate performance of 100 mAh g^-1 at 5C-rate. We anticipate that this discovery will lead to novel mechanisms and material designs for multivalent ion storage, which could open the way to high-energy-density, low-cost and safe batteries.