In this work, we demonstrate for the first time that water-soluble ionic (K + and Na + containing) polymeric carbon nitride (K,Na-PHI) can act as a facile, scalable, and highly effective stabilizer in the synthesis of highly catalytically active ultrafine (< 3 nm) gold nanoparticles. In the selective reduction of 4-nitrophenol to 4-aminophenol by NaBH4 employed as a model catalytic reaction, the K,Na-PHI- stabilized Au NPs not only outperform Au NPs stabilized by monomeric structural units of K,Na-PHI (cyanurate, melonate, cyamelurate), but their activity is even superior (TOF higher by the factor of ~1.6) to that of the benchmark "naked" (i.e., ligand-free) Au NPs electrostatically stabilized by excess NaBH4. These results suggest that K,Na-PHI can provide an effective electrostatic stabilization against the aggregation of Au NPs, without blocking the diffusion of the reactants and products to and from the active sites at the Au catalyst surface. A truly outstanding feature of our K,Na-PHI-stabilized Au NPs is their excellent stability both under ambient conditions (over at least two months), as well as under high ionic strength conditions (up to 0.1 M NaCl solutions). Based on our experimental evidence and theoretical calculations, we propose that the strongly enhanced stability of Au NPs stabilized by K,Na-PHI can be explained by the ability of K,Na-PHI to act as a hydrated protective layer that is, due to the more pronounced delocalization of charges, less prone to the double layer compression effects and thus more effectively diminishes the attractive forces between the nanoparticles. Moreover, a detailed comparative study using different structural units of K,Na-PHI (cyanurate, melonate, cyamelurate) shows that in particular the presence of functionalized heptazine moieties allowing for better charge delocalization is crucial for the synthesis and more effective stabilization of small Au NPs. This is corroborated by theoretical DFT calculations of Fukui functions which reveal the beneficial effect of the counterion-balanced cyamelurate and cyanamide functional groups in the ionic carbon nitride on the orbital alignment and electron delocalization, resulting in more effective stabilization of Au NPs. Thus, this work establishes the ionic carbon nitrides (PHI) as alternative capping agents for nanocatalysts that enable effective stabilization without compromising surface catalysis. More generally, it opens up a route for further developments in utilizing PHI-based stabilizers for the synthesis of high-performance ultrafine nanoparticles, especially for applications in which both the stability and the effective transport of species between the interfacial region of nanoparticles and the environment is mandatory.