Platinum single-atom catalysts (SACs) hold promise as a new frontier in heterogeneous electrocatalysis. However, the exact chemical nature of active Pt sites is highly elusive, arousing many hypotheses to compensate for the significant discrepancies between experiments and theories. Here, we identify the stabilization of low-coordinated PtII species on carbon-based Pt SACs, which have rarely been found as reaction intermediates of homogeneous PtII catalysts, but have often been proposed as catalytic sites for Pt SACs from theory. Advanced online spectroscopic studies reveal multiple identities of PtII moieties on the SACs beyond ideally four-coordinated PtII–N4. Notably, decreasing Pt content to 0.15 wt.% enables the differentiation of low-coordinated PtII species from the four-coordinated ones, demonstrating their critical role in the chlorine evolution reaction (CER). This study suggests general guidelines for achieving high electrocatalytic performance of carbon-based SACs based on all other d8 metal ions, e.g., NiII and PdII as well as PtII.