Interleukin 9 (IL-9) producing helper T (Th9) cells play a crucial role in allergic inflammation, autoimmunity, immunity to extracellular pathogens and anti-tumor immune response. In addition to Th9, Th2, Th17 and Foxp3+ Treg cells produce IL-9. Transcription factor that is critical for IL-9 induction in Th2, Th9 and Th17 cells has not been identified. Here we show that Foxo1, a forkhead family transcription factor, requires for IL-9 induction in Th9 and Th17 cells. We further show that inhibition of AKT enhances IL-9 induction in Th9 cells while it reciprocally regulates IL-9 and IL-17 in Th17 cells via Foxo1. Mechanistically, Foxo1 binds and transactivates IL-9 and IRF4 promoters in Th9, Th17 and iTregs. Furthermore, loss of Foxo1 attenuates IL-9 in mouse and human Th9 and Th17 cells, and ameliorates allergic inflammation in asthma. Our findings thus identify that Foxo1 is essential for IL-9 induction in Th9 and Th17 cells.