Hair straightness/curliness is a highly heritable trait amongst human populations. Previous studies have reported European specific genetic variants influencing hair straightness, but those in East Asians remain unknown. One promising candidate is a derived coding variant of the ectodysplasin A receptor (EDAR), EDARV370A (370A), associated with several phenotypic changes of epidermal appendages. One of the strongest signals of natural selection in human genomes, 370A, has risen to high prevalence in East Asian and Native American populations, whilst being almost absent in Europeans and Africans. This striking frequency distribution and the pleiotropic nature of 370A led us to pursue if hair straightness, another epidermal appendage-related phenotype, is affected by this variant. By studying 1,718 individuals from four distinctive East Asian populations (Han, Tibetan, Mongolian, and Li), we found a significant association between 370A and the straight hair type in the Han (p = 2.90 × 10−6), Tibetan (p = 3.07 × 10−2), and Mongolian (p = 1.03 × 10−5) populations. Combining all the samples, the association is even stronger (p = 5.18 × 10−10). The effect of 370A on hair straightness is additive, with an odds ratio of 2.05. The results indicate very different biological mechanisms of straight hair in Europe and Asia, and also present a more comprehensive picture of the phenotypic consequences of 370A, providing important clues into the potential adaptive forces shaping the evolution of this extraordinary genetic variant.