Significance Th17 cells accumulate in the gut, where they mediate barrier defenses and repair but can also provoke inflammatory disease. In mice, segmented filamentous bacteria (SFB) is sufficient to induce Th17 cells in the gut, but functionally analogous microbes in humans have not been defined. Here, we identified Bifidobacterium adolescentis as one of several human symbiont bacterial species that could, alone, induce Th17 cells in the small intestine of mice. B. adolescentis and SFB exhibited overlapping but also distinct activities, suggesting multiple routes to intestinal Th17 induction. Like SFB, B. adolescentis exacerbated autoimmune arthritis, arguing for its pathological relevance. Our results help to inform the search for therapeutic targets in diseases associated with Th17 responses and mucosal dysfunction.