Objective To investigate the expression and clinical significance of transcription factor SOX5 in peripheral blood mononuclear cells (PBMCs) and serum in patients with rheumatoid arthritis (RA). Methods The relative expression of representative genes of the SOX gene family in the PBMCs from RA patients were detected by Real-Time Polymerase Chain Reaction (RT-PCR), and serum levels of SOX5 expression were measured by enzyme-linked immunosorbent assay (ELISA) in 30 RA patients, 27 osteoarthritis (OA) patients and 30 healthy controls (HC). The expression levels of SOX5 in PBMCs were detected by RT-PCR after stimulated with IL-6, TNF-α, IL-1β and IL-17 for 24 hours. The relationship between SOX5 and receptor activator of nuclear factor κB ligand (RANKL) was detected by co-Immunoprecipitation (co-IP). The formation of TRAP-positive cells after silence SOX5 in osteoclast precursor cell treated with RANKL was observed by Tartrate-resistant acid phasphate stain (TRAP). The differences were tested using one-way ANOVA followed by Student-Newman-Keuls post hoc analysis Correlations were analyzed using Pearson's analysis. Results SOX5 was predominantly expressed in the PBMCs of RA as compared with other SOX family genes in PBMC. PBMC levels of SOX5 in RA patients (21±19) were higher than the OA patients (10±8) and healthy control group (5±4)(F=8.343, P<0.01). While, Serum levels of SOX5 in RA patients [(19132±12054) pg/ml were higher than the OA patients [(9065±15172) pg/ml] and healthy control group [(3242±1251) pg/ml] (F=15.31, P<0.01). IL-6, TNF-α, IL-1β and IL-17 led to the up-regulation of SOX5 expression in PBMCs. IL-6, TNF-α, IL-1β promoted the interaction of SOX5 and RANKL in PBMCs. Silencing SOX5 reduced the formation of TRAP-positive cells in osteoclast precursor cell treated with RANKL. Conclusion Our results have proven that transcriptional factor SOX5 regulates the expression of RANKL and participates in the process of RA bone erosion. Inhibition of SOX5 expression may be a new therapy target of RA.