ABSTRACT The thymus stroma constitutes a fundamental microenvironment for T-cell generation. Despite the chief contribution of thymic epithelial cells, recent studies emphasize the regulatory role of mesenchymal cells in thymic function. Mesenchymal progenitors are suggested to exist in the postnatal thymus; nonetheless, an understanding of their nature and the mechanism controlling their homeostasis in vivo remains elusive. We resolved two new thymic fibroblast subsets with distinct developmental features. Whereas CD140αβ+GP38+SCA-1− cells prevailed in the embryonic thymus and declined thereafter, CD140αβ+GP38+SCA-1+ cells emerged in the late embryonic period and predominated in postnatal life. The fibroblastic-associated transcriptional programme was upregulated in CD140αβ+GP38+SCA-1+ cells, suggesting that they represent a mature subset. Lineage analysis showed that CD140αβ+GP38+SCA-1+ maintained their phenotype in thymic organoids. Strikingly, CD140αβ+GP38+SCA-1− generated CD140αβ+GP38+SCA-1+, inferring that this subset harboured progenitor cell activity. Moreover, the abundance of CD140αβ+GP38+SCA-1+ fibroblasts was gradually reduced in Rag2−/− and Rag2−/−Il2rg−/− thymi, indicating that fibroblast maturation depends on thymic crosstalk. Our findings identify CD140αβ+GP38+SCA-1− as a source of fibroblast progenitors and define SCA-1 as a marker for developmental stages of thymic fibroblast differentiation.