ABSTRACT The exsolution method has garnered significant attention owing to its high efficacy in developing highly efficient and stable metal nanocatalysts. Herein, a versatile exsolution approach is developed to embed size‐tunable metal nanocatalysts within a conductive metal pnictogenide matrix. The gas‐phase reaction of Ru‐substituted Ni–Fe‐layered‐double‐hydroxide (Ni 2 Fe 1− x Ru x ‐LDH) with pnictogenation reagents leads to the exsolution of Ru metal nanocatalysts and a phase transformation into metal pnictogenide. The variation in reactivity of pnictogenation reagents allows for control over the size of the exsolved metal nanocatalysts (i.e., nanoclusters for nitridation and single atoms for phosphidation), underscoring the effectiveness of the pnictogenation‐driven exsolution strategy in stabilizing size‐tunable metal nanocatalysts. The Ru‐exsolved nickel–iron nitride/phosphide demonstrates outstanding electrocatalyst activity for the hydrogen evolution reaction, exhibiting a smaller overpotential and higher stability than Ru‐deposited homologs. The high efficacy of pnictogenation‐assisted exsolution in optimizing the performance and stability of Ru metal nanocatalysts is ascribed to the efficient interfacial electronic interaction between Ru metals and nitride/phosphide ions assisted by the inner sphere mechanism. In situ spectroscopic analyses highlight that exsolved Ru single atoms facilitate more efficient electron transfer to the reactants than the exsolved Ru nanoclusters, which is primarily responsible for the superior impact of the phosphidation‐driven exsolution approach.