Although lithium-ion batteries (LIBs) dominate the market for rechargeable batteries, alternative chemistries are under investigation to develop new batteries, so-called “post-lithium” systems. Among these alternatives, magnesium rechargeable batteries (MRBs) are considered promising, because they use low cost, abundant materials and offer potentially high energy densities. However, there is no information available about the environmental implications associated with the production of these batteries. This chapter identifies and quantifies environmental impacts for a prototype MRB, specifically a magnesium–sulfur battery (Mg–S). The Mg–S cell is comprised of an Mg foil anode, a sulfur cathode, a magnesium tetrakis (hexafluoroisopropyloxy) borate electrolyte, a plastic separator, and aluminum composite pouch cell packaging. The estimation of the environmental impacts is based on a prospective life cycle assessment (LCA) following a cradle-to-gate approach. The life cycle inventory (LCI) of the Mg–S battery is built on lab-scale data, supplemented by information from industrial partners and literature. The indicative results identify the pouch cell housing and electricity requirements for battery manufacture as potential environmental hotspots. The comparison with existing LIB technology shows significant potential for further reduction of the estimated environmental impacts. At present, due to insufficient performance configuration of the prototype cell, the Mg–S battery cannot compete with existing, highly optimized LIBs.