We set out to investigate the influence of STA-21, a dynamic STAT-3 inhibitor, on the expansion and progression of rheumatoid arthritis (RA), and to determine its potential mechanisms of action in a mouse model of collagen antibody-induced arthritis (CAIA). To this end, arthritis was induced via intravenous (IV) injection of Balb/c mice with a cocktail of antibodies directed against type II collagen (1.5μg/mouse, IV), followed by lipopolysaccharide (LPS) at a dose of (25μg/mouse, i.p.) on day 3. Mice were then left untreated or were simultaneously treated with STA-21 (0.5mg/kg, i.p., once daily for 2 weeks) followed by evaluation for clinical and histological features of arthritic inflammation and flow cytometric analysis of cytokines and transcription factors in peripheral blood. STA-21 enhanced the clinical course of arthritis in CAIA mice and decreased CD8⁺RORγt⁺ and CD8⁺IL-21⁺ cells while inducing the production of CD8⁺Foxp3⁺ cells. Furthermore, STA-21 prevented the production of TNF-α and IL-6 in peripheral blood and increased IL-27 production by CD14⁺ cells. Moreover, STA-21 not only regulates Th1/Th2 serum cytokine levels but also the mRNA and protein expression of key factors including NF-κB p65, RORγt, T-bet, IL-4, GATA-3, JAK1, Stat3, and IL-21. Thus, administration of the Stat3 inhibitor STA-21 inhibits cellular signaling pathways and downstream activation of key transcription factors previously shown to play key roles in the pathogenesis of RA. Therefore, these data suggest that STA-21 could be considered as a potential treatment for patients with RA.