Description: Rationale: Intimal calcification is highly correlated with atherosclerotic plaque burden, but the underlying mechanism is poorly understood. We recently reported that cartilage oligomeric matrix protein (COMP), a component of vascular extracellular matrix, is an endogenous inhibitor of vascular smooth muscle cell calcification. Objective: To investigate whether COMP affects atherosclerotic calcification. Methods and Results: ApoE –/– COMP –/– mice fed with chow diet for 12 months manifested more extensive atherosclerotic calcification in the innominate arteries than did ApoE –/– mice. To investigate which origins of COMP contributed to atherosclerotic calcification, bone marrow transplantation was performed between ApoE –/– and ApoE –/– COMP –/– mice. Enhanced calcification was observed in mice transplanted with ApoE –/– COMP –/– bone marrow compared with mice transplanted with ApoE –/– bone marrow, indicating that bone marrow–derived COMP may play a critical role in atherosclerotic calcification. Furthermore, microarray profiling of wild-type and COMP –/– macrophages revealed that COMP-deficient macrophages exerted atherogenic and osteogenic characters. Integrin β3 protein was attenuated in COMP –/– macrophages, and overexpression of integrin β3 inhibited the shift of macrophage phenotypes by COMP deficiency. Furthermore, adeno-associated virus 2–integrin β3 infection attenuated atherosclerotic calcification in ApoE –/– COMP –/– mice. Mechanistically, COMP bound directly to β-tail domain of integrin β3 via its C-terminus, and blocking of the COMP–integrin β3 association by β-tail domain mimicked the COMP deficiency–induced shift in macrophage phenotypes. Similar to COMP deficiency in mice, transduction of adeno-associated virus 2–β-tail domain enhanced atherosclerotic calcification in ApoE –/– mice. Conclusions: These results reveal that COMP deficiency acted via integrin β3 to drive macrophages toward the atherogenic and osteogenic phenotype and thereby aggravate atherosclerotic calcification.