PtO nanodots (similar to 2.5 nm) with high crystallinity are homogeneously deposited on the surface of g-C3N4 by a two-step refluxing-calcination method. XPS analysis indicates that a strong interaction is formed between PtO and g-C3N4, which is beneficial for the transportation of charge carriers from g-C3N4 to the co-catalyst and therefore results in a prolonged carrier life time in the composite. A series of electrochemical tests, including EIS, LSV and OCVD prove that PtO can significantly enhance the conductivity and light response of g-C3N4 compared with Pt. PtO and Serving as hydrogen evolution sites, the highly dispersed PtO nanodots can shorten the diffusion path lengths for photogenerated electrons in the host photocatalyst as well as suppress the undesired hydrogen oxidation reaction. Remarkably, PtO coupled g-C3N4 (0.19 wt% Pt) exhibits excellent hydrogen production activity, giving a turnover frequency of 552.7 h(-1), which is 51 times that of Pt nanoparticles confined g-C3N4. The PtO nanodots also display high photostability and would not agglomerate into clusters or be reduced by the photogenerated electrons even after 40 h of irradiation. The present work also provides a new method for synthesis of high performance co-catalysts with low Pt content used in other photocatalytic reactions and energy applications.