All-solid-state　batteries　(ASSBs)　with　high-energy-density　and　enhanced　safety　are　ideal　for　next-generation　energy　storage　in　electric　transportation　and　Internet　of　Things.　Fundamentally,　the　augmentation　of　their　energy　density　relays　on　advanced　cathode　materials.　This　imperative　has　driven　growing　interest　in　Se-based　cathodes,　which　demonstrate　a　high　volumetric　energy　density,　as　well　as　higher　electrical　conductivity　and　better　environmental　adaptability　compared　to　the　well-known　S　cathodes.　However,　to　ensure　sufficient　mechanical　strength　and　mitigate　the　continuous　deterioration　of　the　solid-solid　interface　caused　by　the　substantial　volume　expansion　of　the　Se,　the　all-solid-state　Li-Se　batteries　reported　thus　far　typically　employ　thick　solid　electrolytes　(50–200 µm),　which　severely　limits　their　energy　density.　Here,　the　first　successful　fabrication　of　all-solid-state　thin-film　Li-Se　batteries　is　reported,　featuring　an　ultra-thin　(≈1.4 µm)　lithium　phosphorus　oxynitride　solid　electrolyte　and　a　hybrid　Se　cathode　supported　by　vertical　graphene　nanoarrays　(VGs).　The　conductive　VGs,　serving　as　the　Se　host,　effectively　mitigate　the　volume　change　during　cycling　and　ensure　stable　solid-solid　contact.　Consequently,　the　cells　exhibit　over　1000　stable　cycles　with　a　capacity　retention　rate　of　89%　are　attained　in　the　＇all-thin　film＇　configuration.　This　study　provides　a　novel　design　strategy　for　the　development　of　next-generation　high-performance　ASSBs.　©　2025　Wiley-VCH　GmbH.