Objective To evaluate the efficacy and safety of adalimumab (ADA) versus hyaluronic acid (HA) by intra-articular injection for moderate to severe knee osteoarthritis. Methods Fifty-six consecutive patients with moderate to severe knee osteoarthritis were randomly allocated to either the ADA group or HA group. On day 0, patients in the ADA group received 10 mg of ADA by intra-articular injection, while those in the HA group received 25 mg of HA. All patients received celecoxib at 200 mg/day for 4 weeks. The pain visual analog scale (VAS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), Patient Global Assessment (PGA), and Physician Global Assessment (PhGA) scores were assessed. Results At baseline, the pain VAS, WOMAC, PGA, and PhGA scores were similar between the two groups. The decrease in the pain VAS score, WOMAC pain score, WOMAC physical function score, and WOMAC total score from baseline to week 4 were greater in the ADA than HA group. A greater decrease in the PGA and PhGA scores from baseline to week 4 was noted in the ADA than HA group. No difference in adverse events was observed between the two groups. Conclusion ADA by intra-articular injection was effective and tolerated for moderate to severe knee osteoarthritis.