The C-type lectin dectin-1 binds to yeast and signals through the kinase Syk and the adaptor CARD9 to induce production of interleukin 10 (IL-10) and IL-2 in dendritic cells (DCs). However, whether this pathway promotes full DC activation remains unclear. Here we show that dectin-1–Syk–CARD9 signaling induced DC maturation and the secretion of proinflammatory cytokines, including IL-6, tumor necrosis factor and IL-23, but little IL-12. Dectin-1-activated DCs 'instructed' the differentiation of CD4⁺ IL-17-producing effector T cells (T_H-17 cells) in vitro, and a dectin-1 agonist acted as an adjuvant promoting the differentiation of T_H-17 and T helper type 1 cells in vivo. Infection with Candida albicans induced CARD9-dependent T_H-17 responses to the organism. Our data indicate that signaling through Syk and CARD9 can couple innate to adaptive immunity independently of Toll-like receptor signals and that CARD9 is required for the development of T_H-17 responses to some pathogens.