Blond hair is a rare human phenotype found almost exclusively in Europe and Oceania. Here, we identify a cystine-to-arginine change at a highly conserved residue in tyrosinase-related protein 1 (TYRP1) as the single source of blond hair in Solomon Islanders. This missense mutation is predicted to impact catalytic activity of the protein and causes blond hair through a recessive mode of inheritance. The novel mutation is at a frequency of 26% in the Solomon Islands but is absent outside of Oceania and represents the largest genetic effect on a visible human phenotype reported to date. Our findings demonstrate that alleles of large effect reach appreciable frequencies in geographically isolated populations and underscore the importance of extending medical genomics to humans worldwide.