AbstractThere are few methods tailored for detecting signals of positive selection in populations directly ancestral to multiple descendent populations. We introduce the ancestral branch statistic (ABS), a four‐population summary statistic for identifying selective sweeps occurring in the direct ancestor of a pair of populations. Simulations show thatABSperforms at least as well as, and often better under model violations, than the complementary likelihood approach of 3P‐CLRacross diverse selection scenarios and parameter values. We first appliedABSto contemporary human genomic data to identify genes that may have been adaptive in ancestral East Asian populations, uncovering the well‐established candidateEDAR, as well as a novel candidateSLC35F3, which encodes a putative thiamine transporter that may have been involved in adaptation to eating polished grains. Next, we performed scans with ancient European genomic data to reexamine evidence of recent positive selection in ancestral Europeans. TheMCM6/LCTcluster and theSLC45A2andHERC2genes are strong outliers, agreeing with previous studies. Novel candidates, such asSLC30A9andCYP1A2, may have been involved in adaptation to local nutrient sufficiency and lifestyle changes. Finally, we provide open‐source software,CalcABS, which can perform genomic scans of ancestral sweeps withABSfrom population allele frequency data.