The　grain　boundary　(GB)　plays　a　crucial　role　in　dominating　hydrogen-induced　plastic　deformation　and　intergranular　failure　in　polycrystal　metals.　In　the　present　study,　molecular　dynamics　simulations　were　employed　to　study　the　effects　of　hydrogen　segregation　on　dislocation　plasticity　of　a　series　of　symmetrical　tilt　grain　boundaries　(STGBs)　with　various　hydrogen　concentrations.　Our　study　shows　that　hydrogen　both　enhances　and　reduces　dislocation　nucleation　events　from　STGBs,　depending　on　different　GB　structures.　Specifically,　for　＜　001　＞　STGBs,　hydrogen　does　not　affect　the　mode　of　heterogeneous　dislocation　nucleation　(HDN),　but　facilitates　nucleation　events　as　a　consequence　of　hydrogen　disordering　the　GB　structure.　Conversely,　hydrogen　retards　dislocation　nucleation　due　to　the　fact　that　hydrogen　segregation　disrupts　the　transformation　of　boundary　structure　such　as　Sigma　9　(2　2　(1)　over　bar)　＜　1　(1)　over　bar0　＞　STGB.　These　results　are　helpful　for　deepening　our　understanding　of　GB-mediated　hydrogen　embrittlement　(HE)　mechanisms.