Excessive reactive oxygen species (ROS) generation is implicated in development of vascular pathology and atherosclerosis, in part, by modulating the phenotype of vascular smooth muscle cells (VSMC). Atherosclerotic plaque VSMC phenotypic transition to macrophage-like cells that express both macrophage and SMC markers is associated with enhanced atherogenesis. We previously reported that NoxA1 deletion decreased ROS levels in VSMC and Noxa1-/-/ApoE-/- mice had attenuated atherosclerosis. In the present study, we investigated the role of VSMC Noxa1-dependent NADPH oxidase activity in the regulation of VSMC phenotypic fate in atherogenesis. Aortic atherosclerotic lesion area was reduced by 36%, lesion volume, determined by the serial sections of the aortic sinus, was reduced by 32% and lesion ROS levels were significantly lower in Apoe-/- mice with SMC-specific Noxa1 deletion (Noxa1SMC-/-) compared with Apoe-/- mice (p<0.01, n=12). Immunofluorescence analysis of aortic sinus sections showed a significant decrease in cells positive for CD68 and myosin11 (22% vs 9%, p<0.05) or Mac3 and smooth muscle actin (17% vs 5%, p<0.01) in the atherosclerotic lesion core of Noxa1SMC-/-/Apoe-/- vs Apoe-/- mice. The expression and nuclear localization of transcription factor KLF4, a modulator of SMC phenotypic transition, and the expression of MCP1, VCAM1, and MMP2 were markedly reduced in the lesions of Noxa1SMC-/-/Apoe-/- vs Apoe-/- mice. VSMC isolated from wild-type mice and treated with oxidized phospholipid POV-PC or IL1β had significantly increased ROS levels and KLF4 protein expression (p<0.01 for both) compared to VSMC from Noxa1SMC-/- mice. In addition, mRNA levels of KLF4-dependent genes - VCAM1, ICAM1, CCL2, and MMP2 - were significantly higher in wild-type compared to cells from Noxa1SMC-/- mice (p<0.05). In conclusion, NADPH oxidase-dependent increase in expression and activity of KLF4 in atherosclerotic lesion VSMC leads to phenotypic transition to macrophage-like cells, increased intraplaque inflammation, matrix remodeling, and lesion volume expansion. Selective inhibition of NOXA1-dependent NADPH oxidase activity in SMC in atherogenesis may be beneficial.