Osteoarthritis (OA) is a major global health problem with no disease-modifying therapies. Interleukin-1 (IL-1) is a critical cytokine associated with the pathophysiology of OA and can be inhibited by IL-1 receptor antagonist (IL-1Ra). Here, we tested the delivery of a gene therapeutic encoding the human IL-1Ra to the knee in a phase 1, open-label clinical trial that enrolled nine patients with radiographic knee OA. The IL-1Ra gene was delivered by a self-complementary (sc)-recombinant adeno-associated virus (rAAV) serotype 2.5 (sc-rAAV2.5IL-1Ra) by intra-articular injection into an index knee at one of three doses: low [1 × 1011 viral genomes (vg)], mid (1 × 1012 vg), or high (1 × 1013 vg). The primary outcome was safety. There were no serious adverse events (AEs) related to sc-rAAV2.5IL-1Ra. Two AEs occurred that were possibly related to the vector. Both were effusions with increased pain and resolved with conservative treatment. sc-rAAV2.5IL-1Ra did not cause changes in vital signs, physical findings, or clinical laboratory measures. Less than 1% of the injected dose of sc-rAAV2.5IL-1Ra vg was detected in circulation after 1 day and was cleared within a week. Titers of neutralizing antibodies to AAV2.5 rose in serum and synovial fluid. In all cases, IL-1Ra concentration increased in the synovial fluid, and IL-1Ra concentrations remained elevated after 1 year. Baseline pain and function scores improved during the study. Therefore, we found that intra-articular gene therapy with sc-rAAV2.5IL-1Ra was safe. The sustained increase in local IL-1Ra in human knee joints supports the further clinical examination of this therapy to provide therapeutic benefit in OA.