Rationally designing cheap and efficient electrocatalysts at the atomic level is highly desirable for the hydrogen evolution reaction (HER). Here, we demonstrate a metallic MoS2 electrocatalyst decorated with platinum single atoms. When combined with electron microscopy observations, our synchrotron X-ray characterizations and theoretical calculations clearly reveal that the doped Pt atoms bond to S atoms on the surface of MoS2. Notably, these Pt single atoms serve as critical active centers for the HER through capturing H+ from the solution. The optimized Pt-MoS2 catalysts achieve significantly enhanced HER performance due to the single-atom coordination effect. This finding is expected to facilitate further realization of hybridized catalysts through the monatomic riveting strategy.