Single‐atom catalysts (SACs) have attracted significant attention due to their superior catalytic activity and selectivity. However, the nature of active sites of SACs under realistic reaction conditions is ambiguous. In this work, high loading Pt single atoms on graphitic carbon nitride (g‐C3N4)‐derived N‐doped carbon nanosheets (Pt1/NCNS) is achieved through atomic layer deposition. Operando X‐ray absorption spectroscopy (XAS) is performed on Pt single atoms and nanoparticles (NPs) in both the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). The operando results indicate that the total unoccupied density of states of Pt 5d orbitals of Pt1 atoms is higher than that of Pt NPs under HER condition, and that a stable Pt oxide is formed during ORR on Pt1/NCNS, which may suppress the adsorption and activation of O2. This work unveils the nature of Pt single atoms under realistic HER and ORR conditions, providing a deeper understanding for designing advanced SACs. Operando X‐ray absorption spectroscopy (XAS) unveils the nature of Pt single atoms under realistic reactions. The total unoccupied density of states of Pt 5d orbitals on Pt1 atoms is higher than that of Pt nanoparticles in a hydrogen evolution reaction, while a stable Pt oxide is formed during an oxygen reduction reaction.