Surface sites of extensive exposed basal planes of MoS₂ nanosheets for the first time have been doped with isolated transition metal atoms as 2D monolayer catalysts for the electrochemical hydrogen evolution reaction (HER). Our HAADF-STEM images clearly show that single transition metal atoms are situated on these multiple surface basal sites that subtly modify the electro-catalytic activities of the monolayer MoS₂ dependent on the electronic and stereospecific properties. From the results of DFT calculations, it is found that these dopers play important roles in tuning the hydrogen adsorption enthalpies on exposed surface S atoms and Mo atoms in HER. Our electrochemical testing, characterization and computational modelling demonstrate that Co can significantly enhance the HER activity by the dominant Co-S interaction whereas Ni substantially lowers the HER rate due to the Ni-Mo interaction on the same basal site despite the two transition metal elements are neighbored to each other in the periodic table. We believe that both the geometric and electronic factors exerted by transition metal dopants are important parameters in further tuning the 2D MoS₂ structure for rational design of the composite materials for more efficient electrochemical hydrogen production from water.