IL-17 is a potent effector cytokine involved in inflammatory response and antimicrobial defense. We report that SIV infection of rhesus macaques (RMs) results in the emergence of IL-17–expressing cells during the acute phase. This subpopulation appears at day 14 postinfection concomitantly with an increase in TGF-β and IL-18 expression. This subset, which exhibits phenotypic markers of NK T cells (NKT), rather than Th17 CD4 cells, persists during the chronic phase and is higher in noncontrollers SIV-infected RMs compared with controllers SIV-infected RMs. In contrast, in the nonpathogenic model of SIVagm infection of African green monkeys, no change in the level of IL-17–expressing cells is observed in lymphoid organs. Consistent with the emergence of TGF-β and IL-18 during the acute phase in SIV-infected RMs, but not in SIV-infected African green monkeys, we demonstrate that in vitro TGF-β and IL-18 induce the differentiation and expansion of IL-17+ NKT+. Altogether, these results demonstrate that IL-17–producing NKT are associated with the pathogenesis of SIV in RMs and suggest that TGF-β and IL-18 play a role in their development.