Background: Leaf rust caused by Puccinia triticina Erikss. is a widely distributed wheat (Triticum aestivum L.) disease. Using wild relatives, such as Triticum spelta L., as a source of desirable traits represents a good strategy for developing wheat varieties, as T. spelta L. has shown tolerance to various types of biotic and abiotic stresses. This study aimed to determine the genetic basis of resistance to leaf rust in the accession Triticum spelta 109 (PI 355580). Methods: The resistant genotype T. spelta 109 was crossed with the bread wheat variety Roelfs F2007, and 135 F3 families were generated to analyze the genetics of resistance to the MBJ/SP leaf rust race. The families were classified into three groups: (i) homozygous-resistant; (ii) homozygous-susceptible; (iii) segregating. A χ2 test was performed to compare whether the expected and observed segregation ratios fit and to determine the number of genes involved in the resistance of T. spelta 109. Results: The seedling tests in the F1 generation showed susceptibility in all plants, indicating that the resistance is conferred by a recessive gene(s). The results of the χ2 test revealed that the observed segregation ratios of the F3 families followed the expected values, suggesting that a recessive gene confers the leaf rust resistance present in T. spelta 109. According to our results and the reported recessive genes identified among the T. spelta accessions, the identified recessive gene in T. spelta 109 (PI355580) is different and most likely a novel leaf rust resistance gene. Conclusions: The genetic resistance to leaf rust of T. spelta 109 (PI 355580) is conferred by a single recessive gene. The importance and usefulness of searching for rust resistance genes from different sources and incorporating them into the genetic base of wheat breeding programs to provide diversity is confirmed.