AbstractAlkali metal‐sulfur batteries (MSBs) are one of the most promising next‐generation energy storage technologies due to their high energy density and potential for low cost. They are nonetheless constrained by the sluggish conversion of metal polysulfides (MPS) during the charge/discharge process. Herein, a multifunctional separator able to trap the MPS and catalyze their conversion in the three main MSB chemistries, Li‐, Na‐, and K‐MSBs, is demonstrated. More in detail, SnSe nanosheets are introduced as additive into the cathode side of the glass microfiber (GF) separator of the MSB. Taking lithium‐sulfur batteries (LSBs) as an example, it is demonstrated that the GF‐SnSe separator (GF@SnSe) shows strong chemical affinity to lithium polysulfides (LiPS) and superior catalytic activity, inhibiting the transport of LiPSs to the anode and accelerating their conversion. Combining experimental and calculation results, the SnSe additive is shown to decrease the Li2S decomposition energy barrier. Overall, GF@SnSe separators provide significantly improved LSB rate performance and cycling stability with a 0.049% capacity decay per cycle. Besides, the GF@SnSe separator promotes the electrochemical performance of sodium‐sulfur and potassium‐sulfur batteries. Overall, this work presents a significant advancement in the development of multifunctional separators in LSBs as well as the emerging Na‐S and K‐S systems.