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Two‒dimensional polymeric carbon nitride (2DPCN) is a versatile support for constructing single atom catalysts (SACs). Herein, we demonstrated a facile strategy for constructing atomic Co sites on 2DPCN. Atomic dispersion of Co was confirmed by the aberration‒corrected high angle annular dark field scanning transmission electron microscopy, while the extended X‒ray absorption fine structure spectroscopy analysis precisely corroborated the existence of atomic Co1–N 4 non square planar sites on 2DPCN support. Remarkably, Co1/2DPCN catalyst yielded H 2 at a rate of 28.3mmol H2 h−1 g Co −1, which is approximately 4.2 times higher than that of CoNPs/2DPCN (6.8mmol H2 h−1g Co −1) counterpart. Notably, with 0.75 wt % Pt as a co‒catalyst, H 2 evolution activity for Co1/2DPCN and CoNPs/2DPCN reached as high as 138.6 mmol H2 h−1 g metal −1 and 22.4mmol H2 h−1 g metal −1, respectively, which could be attributed to the synergistic effect of atomic Co1–N 4 sites and Pt. Both DFT calculations and spectroscopic results revealed that as constructed atomic Co1–N 4 sites, reduced band gap energy, improved light harvesting ability and decreased energy barrier for H 2 evolution. [Display omitted] • Demonstrated facile synthesis of Co single atoms on 2D polymeric carbon nitride support (Co1/2DPCN). • XAS analysis revealed the presence of distinct non square planar Co–N 4 atomic sites. • Atomic Co–N 4 sites promoted light absorption and directional charge transfer. • The atomic Co1/2DPCN catalyst achieved 28.3mmol H2 h−1 g Co −1 activity without the assistant of noble metal co‒catalyst. • Proposed H 2 evolution mechanism on the basis of experimental results and DFT calculations. [ABSTRACT FROM AUTHOR]