Rheumatoid arthritis (RA) is a chronic autoimmune disease with no apparent cure. There is a continued need for the development of new therapeutic strategies. Nanotechnology has been a proven effective route toward novel and fascinating biomedical applications. The application of gold nanoparticles (AuNPs) in RA treatment is still curious. Thus, this study was designed to evaluate the anti-inflammatory and anti-angiogenic potential of spherical AuNPs with different sizes (5, 25, and 75 nm). Collagen-induced arthritis (CIA) rats were divided into CIA model, AuNP, and methotrexate (MTX)-treated groups. Untreated rats were used as the control group. Pro-inflammatory/anti-inflammatory and angiogenic mediators were measured by enzyme-linked immunosorbent assay (ELISA). A remarkable increase in proinflammatory cytokines [IL-6, IL-17, tumor necrosis factor (TNF-α), and interferon (IFN-γ)] coincides with a remarkable reduction in IL-4, IL-10, and transforming growth factor (TGF-β) anti-inflammatory mediators was observed in the CIA group compared to normal animals. A significant increase in vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF) levels was found in the untreated CIA group. Our results confirm the anti-inflammatory properties of AuNPs via downregulating pro-inflammatory (IL-1β, IL-17, IL-23, IL-12, TNF-α, and IFN-γ) and angiogenic mediators (VEGF and PDGF) and increasing anti-inflammatory cytokine (IL-4 and TGF-β) secretion. Our preliminary findings suggest that AuNPs with a diameter of 25 nm could be used as a possible nanotherapeutic agent for RA. In comparison to methotrexate, AuNPs (25 nm) have improved anti-inflammatory/ anti-angiogenic properties in the treatment of experimental rheumatoid arthritis. AuNPs have the potential to be exploited as a novel therapy for RA, bringing a new era in the treatment of autoimmune diseases.