CD4 T cells differentiate into various subsets, including T helper 1 (Th1), Th2, Th9, Th17 and regulatory T (T) cells, which are essential for immune responses and cancer immunotherapy. However, the role of RNA N-methyladenosine (mA) modification in this differentiation is unclear. Here we show that YTHDF2, an important mA reader protein known to destabilize mA-modified mRNA, negatively regulates Th9 cell differentiation. Ablation of Ythdf2 in both mouse and human naive CD4 T cells promotes Th9 differentiation by stabilizing Gata3 and Smad3 mRNA under interleukin-4 (IL-4) and transforming growth factor β (TGF-β) signaling, respectively. Ythdf2-deficient Th9 cells produce increased amounts of IL-9 and IL-21, leading to increased tumor infiltration and cytotoxicity by CD8 T cells and natural killer (NK) cells, thereby improving antitumor activity compared with wild-type Th9 cells. Moreover, YTHDF2 depletion in CAR-Th9 cells enhances their immune activation, reduces their terminal differentiation and augments their antitumor efficacy. Targeting YTHDF2 is thereby a promising strategy to enhance Th9 and CAR-Th9 cell-based cancer immunotherapies.