Dectin-1 signalling in dendritic cells (DCs) has an important role in triggering protective antifungal Th17 responses. However, whether dectin-1 directs DCs to prime antitumour Th9 cells remains unclear. Here, we show that DCs activated by dectin-1 agonists potently promote naive CD4(+) T cells to differentiate into Th9 cells. Abrogation of dectin-1 in DCs completely abolishes their Th9-polarizing capability in response to dectin-1 agonist curdlan. Notably, dectin-1 stimulation of DCs upregulates TNFSF15 and OX40L, which are essential for dectin-1-activated DC-induced Th9 cell priming. Mechanistically, dectin-1 activates Syk, Raf1 and NF-κB signalling pathways, resulting in increased p50 and RelB nuclear translocation and TNFSF15 and OX40L expression. Furthermore, immunization of tumour-bearing mice with dectin-1-activated DCs induces potent antitumour response that depends on Th9 cells and IL-9 induced by dectin-1-activated DCs in vivo. Our results identify dectin-1-activated DCs as a powerful inducer of Th9 cells and antitumour immunity and may have important clinical implications.