SUMMARY Vitamin A and its metabolite, retinoic acid (RA), have recently been implicated in the regulation of immune homeostasis via the peripheral induction of regulatory T cells. Here we show that RA is also required to elicit proinflammatory CD4+ helper T cell responses to infection and mucosal vaccination. Retinoic acid receptor alpha (RARα) is the critical mediator of these effects. Strikingly, antagonism of RAR signaling and deficiency in RARα(Rara−/−) results in a cell autonomous CD4+ T cell activation defect. Altogether, these findings reveal a fundamental role for the RA/RARα axis in the development of both regulatory and inflammatory arms of adaptive immunity and establish nutritional status as a broad regulator of adaptive T cell responses.