Technical Abstract: Race TTKSK (or Ug99) of Puccinia graminis f. sp. tritici possesses virulence on several stem rust resistance genes commonly present in wheat cultivars grown worldwide. New variants detected in the race TTKSK lineage further broadened the virulence spectrum. The identification of sources of genetic resistance to race TTKSK and its relatives is necessary to enable the development and deployment of resistant varieties. Accessions of Triticum monococcum, an A-genome diploid wild and cultivated wheat, have previously been characterized as resistant to stem rust. Three resistance genes were identified and introgressed into hexaploid wheat: Sr21, Sr22, and Sr35. The objective of this study was to determine the genetic control and allelic relationships of resistance to race TTKSK in T. monococcum accessions identified through evaluations at the seedling stage. Generation F2 progeny of 8 crosses between resistant and susceptible accessions and 13 crosses between resistant accessions of T. monococcum were evaluated with race TTKSK and often with North American races including races QFCSC, TTTTF, and MCCFC. For a selected population segregating for three genes conferring resistance to race TTKSK, F2:3 progeny were evaluated with races TTKSK, QFCSC, and TTTTF. We detected at least two genes conferring resistance to race TTKSK that are different from Sr21, Sr22, and Sr35. One of the genes was effective to all races tested. The identification of these genes will facilitate the development of varieties with new resistance to race TTKSK.