The RWA, Diuraphis noxia (Kurdjumov), is a devastating insect pest of wheat (Triticumaestivum L.) and barley (Hordeumvulgare) in the United States and in many parts of the world. The use of D. noxia-resistant cultivars is an economically useful approach for protecting cereals from this aphid. However, there are few genes conferring resistance to the most predominant US biotype (Biotype RWA2). Wheat line CI2401, originating from Tajikistan, has been identified to be resistant to RWA2. An F2-derived F3 (F2:3) segregating population developed from a cross between CI2401 and Glupro (a high quality susceptible wheat cultivar) was used to genetically map the resistance in CI2401. Seedlings from F2 individuals and F3 families were infested with RWA2 aphids. Seedling reactions were scored as resistant or susceptible based on the degrees of leaf rolling and chlorosis. The observed segregation ratios in the F2 and F3 generations indicate the presence of a major dominant gene controlling resistance to RWA2. The gene, named Dn2401, was genetically mapped to the short arm of chromosome 7D. Xbarc214 mapped 1.1 cM and Xgwm473 mapped 1.8 cM distal and proximal, respectively, to the gene. Association studies using more than 12,000 SNPs and SilicoDArTs confirmed the presence of a major signal associated with resistance on chromosome 7DS. In addition, a minor signal was detected in chromosome 1D. The markers developed in this study will be useful for marker-assisted-breeding for resistance to RWA2.