CD9 is a transmembrane glycoprotein belonging to the tetraspanin family. CD9 expression has been reported to be associated with cellular signaling, cell adhesion, cell migration, and tumor related processes. The aim of this study was to examine the immunohistochemical expression of CD9 in vascular senescence and atherosclerosis. One hundred and twenty samples of normal young arteries (obtained from individuals aged 0-60 years), 40 samples of normal old arteries (obtained from individuals aged 61-80 years), and 67 samples of atherosclerotic arteries were obtained from surgically resected specimens. Tissue microarray blocks were prepared for immunohistochemical staining. Immunohistochemical staining detected CD9 expression in 10.8% (13 of 120 samples) of normal young arteries and 30.0% (12 of 40 samples) of normal old arteries. CD9 expression was absent or mildly present in the smooth muscle cells and endothelial cells of normal arteries. Normal old arteries showed significantly higher expression of CD9 than normal young arteries (P<0.01). Atherosclerotic arteries showed moderate or strong CD9 expression (65 of 67 samples, 97.0%), which was observed in the smooth muscle cells, endothelial cells, macrophages, and atheromatous plaques. CD9 was significantly expressed in the atherosclerotic arteries compared to normal young and old arteries (P<0.01). The results suggest that CD9 expression may play an important role in the vascular senescence and pathogenesis of atherosclerosis.