Systemic sclerosis (SSc) is an autoimmune disease characterized by vascular damage and fibrosis of the skin and internal organs. Because activated and oligoclonally expanded CD8+ T cells can be detected in peripheral blood and lungs of SSc patients, effector memory CD8+ T cells may play a critical role for organ involvement in SSc; however, the pathogenic functions of effector memory CD8+ T cells remain incompletely understood. In this study, we performed DNA microarray analysis of the sort-purified effector memory CD8+ T cells from SSc patients and healthy controls, and showed that the expression of genes related to immune response and cell adhesion, including CD226 (also known as DNAX accessory molecule-1 [DNAM-1]), was significantly altered. Moreover, detailed analysis of CD226 revealed that CD226 high CD8+ T cells were increased in SSc patients (mean, 50.7%) compared with healthy controls (32.9%) and were appreciably associated with the severity of skin sclerosis and interstitial lung disease. Furthermore, CD226+ CD8+ T cells produced higher amount of various cytokines than CD226− ones, and CD226 high CD8+ T cells from SSc patients showed upregulated IL-13 production and positive correlation with the cytotoxic capacity of CD8+ T cells against HUVECs. Finally, the neutralization of CD226 in CD8+ T cells impaired costimulation, cytokine productions, and cytolysis against HUVECs. These findings indicate that upregulated CD226 expression on CD8+ T cells reflects disease severity and is involved in SSc pathogenesis via the production of various cytokines, including profibrotic IL-13 and endothelial cell injury, and that CD226 may be a useful target in the treatment of SSc.