Inflammatory and anti-inflammatory cytokines play an important role in the generation of effector and memory CD8+ T cells. We used two different models, transgenic expression of truncated (dominant negative) form of TGF-βRII (dnTGFβRII) and Cre-mediated deletion of the floxed TGF-βRII to examine the role of TGF-β signaling in the formation, function, and homeostatic proliferation of memory CD8+ T cells. Blocking TGF-β signaling in effector CD8+ T cells using both of these models demonstrated a role for TGF-β in regulating the number of short-lived effector cells but did not alter memory CD8+ T cell formation and their function upon Listeria monocytogenes infection in mice. Interestingly, however, a massive lymphoproliferative disorder and cellular transformation were observed in Ag-experienced and homeostatically generated memory CD8+ T cells only in cells that express the dnTGFβRII and not in cells with a complete deletion of TGF-βRII. Furthermore, the development of transformed memory CD8+ T cells expressing dnTGFβRII was IL-7–and IL-15–independent, and MHC class I was not required for their proliferation. We show that transgenic expression of the dnTGFβRII, rather than the absence of TGF-βRII–mediated signaling, is responsible for dysregulated expansion of memory CD8+ T cells. This study uncovers a previously unrecognized dominant function of the dnTGFβRII in CD8+ T cell proliferation and cellular transformation, which is caused by a mechanism that is different from the absence of TGF-β signaling. These results should be considered during both basic and translational studies where there is a desire to block TGF-β signaling in CD8+ T cells.