How naive CD4⁺ T cells commit to the T helper type 2 (T_H2) lineage is poorly understood. Here we show that the basic helix-loop-helix transcription factor Dec2 was selectively expressed in T_H2 cells. CD4⁺ T cells from Dec2-deficient mice showed defective T_H2 differentiation in vitro and in vivo in an asthma model and in response to challenge with a parasite antigen. Dec2 promoted expression of interleukin 4 (IL-4), IL-5 and IL-13 during early T_H2 differentiation and directly bound to and activated transcription of genes encoding the transcription factors JunB and GATA-3. As GATA-3 induces Dec2 expression, our findings also indicate a feed-forward regulatory circuit during T_H2 differentiation.