The peptidylprolyl isomerase Pin1 is over-expressed in some human diseases including malignancies and chronic inflammatory diseases, this suggests that it contributes to the constitutive activation of certain intracellular signaling pathways that promote cell proliferation and cell invasion. Here, we investigate the possible role of Pin1 in rheumatoid arthritis (RA). Pin1 expression was immunohistochemically analyzed in synovial tissue (ST) obtained from patients with RA and osteoarthritis (OA). To investigate the correlation between Pin1 and motility and proliferation of synovial cells, Pin1 localization was immunohistochemically compared with matrix metalloproteinase (MMP)-1, MMP-3, and proliferating cell nuclear antigen (PCNA). Double immunofluorescent staining for Pin1 and p65 was performed to determine whether Pin1 is involved in nuclear factor κB (NF-κB) activation in RA-ST. Results showed Pin1 expression was significantly higher in RA-ST than in OA-ST. The expression of MMP-1, MMP-3, and PCNA was also significantly elevated in RA-ST. Double immunofluorescent staining revealed colocalization of Pin1 and p65 in the nuclei of RA-ST. These results suggest that Pin1 may be involved in the pathogenesis of RA binding with p65 to activate the proteins MMP-1, MMP-3, and PCNA. Therefore, Pin1 may play a pivotal role in the pathogenesis of RA.