Aims: Increased oxidative stress and vascular inflammation are implicated in increased cardiovascular disease (CVD) incidence with age. We and others demonstrated that NOX1/2 NADPH oxidase inhibition, by genetic deletion of p47phox, in Apoe−/− mice decreases vascular reactive oxygen species (ROS) generation and atherosclerosis in young age. The present study examined whether NOX1/2 NADPH oxidases are also pivotal to aging-associated CVD. Results: Both aged (16 months) Apoe−/− and Apoe−/−/p47phox−/− mice had increased atherosclerotic lesion area, aortic stiffness, and systolic dysfunction compared with young (4 months) cohorts. Cellular and mitochondrial ROS (mtROS) levels were significantly higher in aortic wall and vascular smooth muscle cells (VSMCs) from aged wild-type and p47phox−/− mice. VSMCs from aged mice had increased mitochondrial protein oxidation and dysfunction and increased vascular cell adhesion molecule 1 expression, which was abrogated with (2-(2,2,6,6-Tetramethylpiperidin-1-oxyl-4-ylamino)-2-oxoethyl)triphenylphosphonium chloride (MitoTEMPO) treatment. NOX4 expression was increased in the vasculature and mitochondria of aged mice and its suppression with shRNA in VSMCs from aged mice decreased mtROS levels and improved function. Increased mtROS levels were associated with enhanced mitochondrial NOX4 expression in aortic VSMCs from aged subjects, and NOX4 expression levels in arterial wall correlated with age and atherosclerotic severity. Aged Apoe−/− mice treated with MitoTEMPO and 2-(2-chlorophenyl)-4-methyl-5-(pyridin-2-ylmethyl)-1H-pyrazolo[4,3-c]pyridine-3,6(2H,5H)-dione had decreased vascular ROS levels and atherosclerosis and preserved vascular and cardiac function. Innovation and Conclusion: These data suggest that NOX4, but not NOX1/2, and mitochondrial oxidative stress are mediators of CVD in aging under hyperlipidemic conditions. Regulating NOX4 activity/expression and using mitochondrial antioxidants are potential approaches to reducing aging-associated CVD. Antioxid. Redox Signal. 23, 1389–1409.