Recently, we reported the first synthesis of fully water-soluble ionic polymeric carbon nitride, i.e., potassium and sodium ion-containing poly(heptazine imide) (K,Na-PHI), an ionic member of the polymeric carbon nitride family. In aqueous solutions, the K,Na-PHI material can be fully dispersed in form of very small K,Na-PHI colloidal nanoparticles (hydrodynamic diameter ∼10 nm ) containing a large number of cyanamide and cyamelurate surface moieties, resulting in highly stable quasihomogeneous solutions of K,Na-PHI with no gelation or precipitation over several months. Given the unique features of K,Na-PHI (i.e., water solubility, chemical stability, and ionic nature), we hypothesized that water-soluble K,Na-PHI might open a completely new and attractive possibility to stabilize catalytically active metal NPs. While various promising properties of ionic carbon nitrides, such as their photocatalytic activity and charge storage properties, are well established, the use of ionic carbon nitrides as stabilizers has not been reported so far. In this contribution, we demonstrate, for the first time, the ability of K,Na-PHI to effectively stabilize ultrafine Au NPs (<3 nm).