The　structures　and　electronic　properties　of　diverse　transition　metal　(TM = Fe,　Co,　Ni,　Cu,　Ag,　Au,　Rh,　Pd,　Pt　and　Ir)-embedded　monolayer　MoS2　in　the　S-vacancy　and　the　adsorption　of　various　gas　molecules　(CO,　NO,　O2,　NO2,　NH3)　have　been　systematically　investigated　using　density　functional　theory　(DFT).　Our　results　show　that　strong　interactions　between　the　different　transition　metal　atoms　and　S　defect　site　in　monolayer　MoS2　suggest　that　the　transition　metal-embedded　monolayer　MoS2　should　be　stable.　The　embedded　transition　metal　atoms　can　effectively　modulate　the　electronic　structures　and　magnetic　properties　of　inert　monolayer　MoS2,　and　drastically　enhance　the　adsorption　and　the　activation　of　CO,　NO,　O2,　NO2　and　NH3　gas　molecules,　with　respect　to　that　on　pristine　monolayer　MoS2　with　weak　physisorption.　The　electronic　structure　analysis　reveals　that　introduced　nd　orbital　impurity　states　of　embedded　transition　metals　in　the　band　gap　play　an　important　role　in　the　adsorption　and　effectively　activation　of　gas　molecules.　In　addition,　obvious　charge　transfer　occur　from　TM-MoS2　to　oxidizing　gas　molecules　(NO2,　O2)　acting　as　acceptors,　whereas　the　direction　of　charge　transfer　is　reversed　for　the　adsorption　of　the　reducing　gas　(NH3)　as　donor.　Particularly,　among　all　transition　metal　elements,　embedded　Fe　and　Co　have　better　thermal　stability,　adsorption　properties　and　chemical　activities　for　gas　molecules　adsorption.　Our　results　open　new　perspectives　for　the　design　and　the　study　of　high　active　MoS2-based　2D-nanomaterials.　©　2017　Elsevier　B.V.