Invariant natural killer T (iNKT) cells constitute a distinct subset of T lymphocytes exhibiting important immune-regulatory functions. Although various steps of their differentiation have been well characterized, the factors controlling their development remain poorly documented. Here, we show that TGF-β controls the differentiation program of iNKT cells. We demonstrate that TGF-β signaling carefully and specifically orchestrates several steps of iNKT cell development. In vivo, this multifaceted role of TGF-β involves the concerted action of different pathways of TGF-β signaling. Whereas the Tif-1γ branch controls lineage expansion, the Smad4 branch maintains the maturation stage that is initially repressed by a Tif-1γ/Smad4-independent branch. Thus, these three different branches of TGF-β signaling function in concert as complementary effectors, allowing TGF-β to fine tune the iNKT cell differentiation program.