This　study　aimed　at　determining　the　feasibility　of　using　protonated　spent　mushroom　substrate　(PSMS),　an　abundant　agricultural　waste　in　China,　as　a　low　cost　biosorbent　for　the　removal　of　Reactive　Red　15　(RR15)　from　aqueous　solutions.　Batch　biosorption　studies　were　conducted　to　evaluate　the　effects　of　pH　value,　biosorbent　dosage,　initial　dye　concentration　and　contact　time　on　the　removal　of　RR15.　The　results　show　that　initial　pH　would　be　the　most　important　parameter　affecting　RR15　biosorption,　and　the　optimal　pH　was　1.2.　Freundlich　isotherm　fitted　well　with　the　equilibium　data.　Kinetic　studies　show　that　biosorption　of　RR15　followed　the　pseudo-second-order　kinetic　model,　both　intraparticle　diffusion　and　boundary　layer　diffusion　might　affect　the　biosorption　rate.　The　thermodynamic　analysis　indicates　that　the　biosorption　process　is　spontaneous　and　exothermic.　The　FTIR　analysis　reveals　that　chemical　functional　groups　(e.g.,　amine,　phosphonate　and　others)　on　PSMS　would　be　the　active　binding　sites　for　biosorption　of　PSMS.　These　results　show　that　PSMS　could　be　employed　is　a　low-cost　and　alternative　biosorbent　for　the　removal　of　RR15　from　wastewater.