Practical implementation of CH2Br2 hydrodebromination to CH3Br, an important step in the bromine-mediated functionalization of methane, is currently hindered by the lack of selective and stable catalysts, which typically deactivate over a few hours on stream. Despite ranking as the most active metal for this reaction, palladium has received limited attention due to the tendency of metal nanoparticles to form C2+ products and methane, alongside CH3Br. Herein, we explore metal nuclearity and host effects in nanostructured palladium-based hydrodebromination catalysts. While Pd nanoparticles intrinsically exhibit similar unselective behavior irrespective of the host choice, encompassing metal oxides and carbon-based supports, stabilization of isolated Pd sites over the latter enables suppression of C–C coupling. Therein, the impact of host functionalization and structure on the metal center reactivity is investigated over amorphous activated and nitrogen-doped carbons as well as crystalline carbon nitrides. By maximizing the site architectural uniformity and integrating large N-cavities with high affinity for the metal, we show that high-valent Pd single atoms supported on a poly(triazine imide) carbon nitride unlocks full selectivity to CH3Br (99%) and stable behavior over 24 h on stream. Finally, by combining spectroscopic, kinetic, and computational analyses, we identify that the oxidation state of the metal centers, regulated by the coordination with the host, dictates their thermodynamic propensity to selectively hydrogenate the CH2Br* intermediate to CH3Br.