Alkali　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.　©　2024　Wiley-VCH　GmbH.