The molecular mechanisms that direct the development of TCRαβ⁺CD8αα⁺ intestinal intraepithelial lymphocytes (IELs) are not thoroughly understood. Here we show that transforming growth factor-β (TGF-β) controls the development of TCRαβ⁺CD8αα⁺ IELs. Mice with either a null mutation in the gene encoding TGF-β1 or T cell–specific deletion of TGF-β receptor I lacked TCRαβ⁺CD8αα⁺ IELs, whereas mice with transgenic overexpression of TGF-β1 had a larger population of TCRαβ⁺CD8αα⁺ IELs. We observed defective development of the TCRαβ⁺CD8αα⁺ IEL thymic precursors (CD4⁻CD8⁻TCRαβ⁺CD5⁺) in the absence of TGF-β. In addition, we found that TGF-β signaling induced CD8α expression in TCRαβ⁺CD8αα⁺ IEL thymic precursors and induced and maintained CD8α expression in peripheral populations of T cells. Our data demonstrate a previously unrecognized role for TGF-β in the development of TCRαβ⁺CD8αα⁺ IELs and the expression of CD8α in T cells.