A gene for Ug99 resistance from wheat landrace CItr 4311 was detected on the long arm of chromosome 2B. Wheat landrace CItr 4311 has seedling resistance to stem rust caused by Puccinia graminis f. sp. tritici race TTKSK and field resistance to the Ug99 race group. Parents, F1 seedlings, 121 doubled haploid (DH) lines, and 124 recombinant inbred lines (RILs) developed from a cross between CItr 4311 and the susceptible line LMPG-6 were evaluated for seedling resistance to race TTKSK. Goodness-of-fit tests indicated that a single dominant gene in CItr 4311 conditioned the TTKSK resistance. The 90 K wheat iSelect SNP platform was used to genotype parents and the DH population. The seedling resistance locus was mapped to the chromosome arm 2BL. Parents and the DH population were evaluated for field resistance in Kenya. One major QTL for the field resistance was consistently detected in the same region on 2BL as the seedling resistance. Using KASP assays, five linked SNP markers were used to verify the result in the 124 RIL, 35 wheat accessions, 46 DH lines from the LMPG-6/PI 165194 cross and F1 seedlings, and susceptible bulks derived from crosses between six resistant landraces with LMPG-6. Race specificity, mapping results, and haplotype similarity with lines with Sr9h (Gabo 56, Timstein, and PI 670015), support the hypothesis that the Sr gene in CItr 4311 and the landraces is Sr9h. The KASP assays developed in this study will be useful for pyramiding the TTKSK resistance from CItr 4311 with other Sr genes effective against Ug99.