IL-33 has recently been identified as a cytokine endowed with pro-Th2 functions, raising the question of its effect on invariant natural killer T cell (iNKT), which are potent IL-4 producers. Here, we report a two-fold increase of iNKT-cell counts in spleen and liver after a 7-day treatment of mice with IL-33, which results from a direct effect, given that purified iNKT cells express the T1/ST2 receptor constitutively and respond to IL-33 by in vitro expansion and functional activation. Conversely to the expected pro-Th2 effect, IL-33 induced a preferential increase in IFN-gamma rather than IL-4 production upon TCR engagement that depended on endogenous IL-12. Moreover, in combination with the pro-inflammatory cytokine IL-12, IL-33 enhanced IFN-gamma production by both iNKT and NK cells. Taken together these data support the conclusion that IL-33 can contribute as a co-stimulatory factor to innate cellular immune responses.