The hydrogen evolution reaction (HER) is known to be influenced by the identity of alkali metal cations in the electrolyte. But a clear understanding of this behavior has not been developed. Here, we present the results of experimental and theoretical studies that describe how alkali metal cations influence the HER in acidic and basic electrolytes. Alkali metal cations are shown to have no systematic effect on HER rates in acid. In alkaline media, rates decrease with increasing cation size over Ir, Pd, and Pt (Li+ > Na+ > K+ > Cs+) and increase with cation size over Cu, Ag, and Au (Li+ < Na+ < K+ < Cs+). We argue that interfacial cations lower the activation barrier for water dissociation, an elementary step unique to the HER in alkaline media. HER rates are limited by this step on Cu, Ag, and Au but are not over Ir, Pd, and Pt, explaining the inverted activity trend. Based on ab initio molecular dynamics simulations, we suggest that trends with cation size are attributable to the greater willingness of large, weakly solvated cations to approach the electrode surface.