Platinum single-atom catalysts 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 PtIIspecies on carbon-based Pt single-atom catalysts, which have rarely been found as reaction intermediates of homogeneous PtIIcatalysts but have often been proposed as catalytic sites for Pt single-atom catalysts from theory. Advanced online spectroscopic studies reveal multiple identities of PtIImoieties on the single-atom catalysts beyond ideally four-coordinated PtII–N4. Notably, decreasing Pt content to 0.15 wt.% enables the differentiation of low-coordinated PtIIspecies from the four-coordinated ones, demonstrating their critical role in the chlorine evolution reaction. This study may afford general guidelines for achieving a high electrocatalytic performance of carbon-based single-atom catalysts based on otherd8metal ions.