In　this　study,　graphitic　carbon　nitride　supported　platinum　nanocomposites　(g-C3N4/PtNPs)　have　been　synthesized　for　the　first　time　by　an　ultrasonic-assisted　chemical　reduction　method.　By　using　g-C3N4　as　the　stabilizer,　Pt　ions　could　be　reduced　to　PtNPs　by　NaBH4　and　uniformly　deposited　on　the　surface　of　g-C3N4.　The　resulting　g-C3N4/PtNPs　exhibited　enhanced　catalytic　activity　for　the　oxidation　of　3,3′,5,5’-tetramethylbenzidine　(TMB)　compared　to　that　of　g-C3N4　or　PtNPs　alone.　After　interaction　with　Hg2+,　the　catalytic　activity　of　g-C3N4/PtNPs　was　effectively　inhibited　due　to　the　formation　of　HgPt　amalgam.　On　the　basis　of　this　effect,　a　novel　label　free　colorimetric　sensor　has　been　developed　for　sensitive　detection　of　Hg2+　through　the　g-C3N4/PtNPs　mediated　catalytic　reaction.　A　detection　limit　as　low　as　1.23　nM　was　achieved.　This　assay　also　exhibited　excellent　selectivity　toward　Hg2+　over　other　metal　ions.　In　addition,　it　was　successfully　applied　to　the　determination　of　Hg2+　in　real　water　samples.　In　view　of　the　advantages,　such　as　simple　operation,　cost-effective,　rapid　response　and　naked-eye　observation,　the　developed　colorimetric　sensor　hold　great　potential　for　the　detection　of　toxic　Hg2+　in　environmental　and　biological　samples.　©　2017　Elsevier　B.V.