How the development of antibacterial T helper 17 (Th17) cells is selectively promoted by antigen-presenting dendritic cells (DCs) is unclear. We showed that bacteria, but not viruses, primed human DCs to promote IL-17 production in memory Th cells through the nucleotide oligomerization domain 2 (NOD2)-ligand muramyldipeptide (MDP), a derivative of bacterial peptidoglycan. MDP enhanced obligate bacterial Toll-like receptor (TLR) agonist induction of IL-23 and IL-1, which promoted IL-17 expression in T cells. The role of NOD2 in this IL-23-IL-1-IL-17 axis could be confirmed in NOD2-deficient DCs, such as DCs from selected Crohn's disease patients. Thus, antibacterial Th17-mediated immunity in humans is orchestrated by DCs upon sensing bacterial NOD2-ligand MDP.