BACKGROUND Rheumatoid arthritis is an inflammatory autoimmune disorder. Withania somnifera Dunal (Solanaceae) (WS), is a common medicinal plant used in traditional systems of medicine for the treatment of arthritis, and is an ingredient of anti-arthritic polyherbal formulations such as Habb-e-Asgand® and Arthritin™. In the present study, we evaluated the antioxidant and anti-arthritic activity of aqueous extract of WS root (WSAq) in collagen-induced arthritic (CIA) rats. METHODS CIA rats were treated by using three doses of WSAq (100, 200, 300 mg/kg b. wt., orally) and methotrexate (MTX, 0.25 mg/kg b. wt. i.p.) as a standard reference drug for 20 days. The anti-arthritic effect was assayed by measuring the arthritic index, autoantibodies such as rheumatoid factor (RF), anti-cyclic citrullinated peptide antibody (a-CCP), anti-nuclear antibody (ANA), anti-collagen type II antibody (a-CII) and inflammatory marker like C-reactive protein (CRP). The oxidative stress parameters were also measured. RESULTS Treatment with WSAq resulted in a dose-dependent reduction in arthritic index, autoantibodies and CRP (p < 0.05) with maximum effect at dose of 300 mg/kg b. wt. and the results were comparable to that of MTX-treated rats. Similarly, oxidative stress in CIA rats was ameliorated by treatment with different doses of WSAq, as evidenced by a decrease in lipid peroxidation and glutathione-S-transferase activity and an increase in the glutathione content and ferric-reducing ability of plasma (p < 0.05). CONCLUSIONS The results showed that WSAq exhibited antioxidant and anti-arthritic activity and reduced inflammation in CIA rats and suggests the potential use of this plant in the treatment of arthritis.