<p>Lanthanide (Ln) elements have distinctive electronic structures and chemical behaviours that can be used to tune electrocatalytic performance when they are introduced as isolated atomic modifiers. However, their broader use remains limited because their high reactivity and ultralow reduction potentials make it difficult to develop general synthesis strategies that can atomically disperse Ln atoms on diverse substrates. Here we develop a molten-nitrite method that yields Ln single-atom catalysts, permitting the atomic isolation of multiple lanthanides on various supports, including metals, metal oxides and carbon materials. Mechanistic insights obtained from systematic control experiments indicate that Ln single-atom catalyst formation in molten nitrites is dictated by three factors: the Lux–Flood basicity effect, mass-diffusion resistance and molten-salt shielding. As a demonstration, Dy₁/Pt shows an overpotential of 20 mV at a current density of −10 mA cm⁻² in 0.5-M H₂SO₄ for acidic hydrogen evolution, which is superior to commercial Pt/C catalysts. This work establishes a framework for synthesizing Ln single-atom catalysts and positions molten-nitrite systems as a versatile platform for electrocatalyst synthesis.</p>