Eco-friendly　lead-free　organic-inorganic　manganese　halides　(OIMHs)　have　attracted　considerable　attention　in　various　optoelectronic　applications　because　of　their　superior　optical　properties　and　flexible　solution　processibility.　Herein,　we　report　a　novel　pseudo-2D　layered　OIMH　(MTP)2MnBr4　(MTP:　methyltriphenylphosphonium),　which　exhibits　intense　green　emission　under　UV/blue　or　X-ray　excitation,　with　a　near-unity　photoluminescence　quantum　yield,　high　resistance　to　thermal　quenching　(I150　degrees　C=84.1　%)　and　good　photochemical　stability.　These　features　enable　(MTP)2MnBr4　as　an　efficient　green　phosphor　for　blue-converted　white　light-emitting　diodes,　demonstrating　a　commercial-level　luminous　efficiency　of　101　lm　W-1　and　a　wide　color　gamut　of　116　%　NTSC.　Moreover,　these　(MTP)2MnBr4　crystals　showcase　outstanding　X-ray　scintillation　properties,　delivering　a　light　yield　of　67000　photon　MeV-1,　a　detection　limit　of　82.4　nGy　s-1,　and　a　competitive　spatial　resolution　of　6.2　lp　mm-1　for　X-ray　imaging.　This　work　presents　a　new　avenue　for　the　exploration　of　eco-friendly　luminescent　OIMHs　towards　multifunctional　light-emitting　applications.　A　novel　pseudo-2D　layered　organic-inorganic　manganese　bromide　with　a　near-unity　photoluminescence　quantum　yield　(PLQY)　and　high　thermal　quenching　resistance　was　developed　and　explored　as　a　green　phosphor　for　wide　color　gamut　light-emitting　diodes　(LEDs)　and　as　a　scintillator　for　sensitive　X-ray　detection　and　imaging,　with　performance　superior　to　that　of　most　commercial　phosphors　and　scintillators,　respectively.image