T helper 17 (T(H)17) lymphocytes protect mucosal barriers from infections, but also contribute to multiple chronic inflammatory diseases. Their differentiation is controlled by ROR gamma t, a ligand-regulated nuclear receptor. Here we identify the RNA helicase DEAD-box protein 5 (DDX5) as a ROR gamma t partner that coordinates transcription of selective T(H)17 genes, and is required for T(H)17-mediated inflammatory pathologies. Surprisingly, the ability of DDX5 to interact with ROR gamma t and coactivate its targets depends on intrinsic RNA helicase activity and binding of a conserved nuclear long noncoding RNA (lncRNA), Rmrp, which is mutated in patients with cartilage-hair hypoplasia. A targeted Rmrp gene mutation in mice, corresponding to a gene mutation in cartilage-hair hypoplasia patients, altered lncRNA chromatin occupancy, and reduced the DDX5-ROR gamma t interaction and ROR gamma t target gene transcription. Elucidation of the link between Rmrp and the DDX5-ROR gamma t complex reveals a role for RNA helicases and lncRNAs in tissue-specific transcriptional regulation, and provides new opportunities for therapeutic intervention in T(H)17-dependent diseases.