Race TTKSK (or Ug99) of Puccinia graminis f. sp. tritici, the causal agent of wheat stem rust, is a serious threat to wheat production worldwide. Diploid wheat, Triticum monococcum (genome Am), has been utilized previously for the introgression of stem rust resistance genes Sr21, Sr22, and Sr35. Multipathotype seedling tests of biparental populations demonstrated that T. monococcum accession PI 306540 collected in Romania contains a recessive resistance gene effective to all P. graminis f. sp. tritici races screened, including race TTKSK. We will refer to this gene as SrTm4, which is the fourth stem rust resistance gene characterized from T. monococcum. Using two mapping populations derived from crosses of PI 272557×PI 306540 and G3116×PI 306540, we mapped SrTm4 on chromosome arm 2AmL within a 2.1 cM interval flanked by sequence-tagged markers BQ461276 and DR732348, which corresponds to a 240-kb region in Brachypodium chromosome 5. The eight microsatellite and nine sequence-tagged markers linked to SrTm4 will facilitate the introgression and accelerate the deployment of SrTm4-mediated Ug99 resistance in wheat breeding programs.