Synthesis of single-atom catalysts (SACs) remains challenging, particularly to achieve high metal loading while preserving atomic dispersion. Here, we report a low-temperature synthesis strategy that effectively stabilizes individual single metal atoms on a 2D stacked polytriazine framework (g-C3N4) with a high metal loading. In this context, a microwave-assisted method was utilized for the synthesis of Ni-based SACs embedded in 2D stacked polytriazine at 140 °C for 30 min. A thorough analysis of XRD, XPS, and XAS reveals the co-ordination of Ni with N in the polytriazine structure, which not only facilitates stacking of polytriazine sheets but also helps in the stabilization of Ni single atoms. Both density functional theory (DFT) calculation and EXAFS curve fittings reveal that Ni2+ is present in-plane being coordinated with four N atoms and between two layers of the polytriazine framework in co-ordination with six N atoms. The resulting SAC achieves a Ni metal loading of up to 2.8 wt %, with a specific surface area (SABET) of 418 m2 g–1. This system exhibits not only Ni single atomic active sites but also strong Ni–N co-ordination, which could be effective for electrochemical oxygen evolution reaction (OER). Further, the SAC demonstrates high electrocatalytic performance toward OER, with a low overpotential of 330 mV at 10 mA cm–2 and a low Tafel slope of 84 mV dec–1 in 1 M KOH. The combined effect of both Ni single atomic active sites and stacked polytriazine leads to a high turnover frequency (TOF) of 0.05 s–1 and stability up to 50 h without any appreciable change in the microstructure.