Thoracic and abdominal aortic aneurysm poses a substantial mortality risk in adults, yet many of its underlying factors remain unidentified. Here, we identify mitochondrial nicotinamide adenine dinucleotide (NAD)⁺ deficiency as a causal factor for the development of aortic aneurysm. Multiomics analysis of 150 surgical aortic specimens indicated impaired NAD salvage and mitochondrial transport in human thoracic aortic aneurysm, with expression of the NAD transporter SLC25A51 inversely correlating with disease severity and postoperative progression. Genome-wide gene-based association analysis further linked low SLC25A51 expression to risk of aortic aneurysm and dissection. In mouse models, smooth muscle-specific knockout of Nampt, Nmnat1, Nmnat3, Slc25a51, Nadk2 and Aldh18a1, genes involved in NAD salvage and transport, induced aortic aneurysm, with Slc25a51 deletion producing the most severe effects. Using these models, we suggest a mechanism that may explain the disease pathogenesis: the production of type III procollagen during aortic medial matrix turnover imposes a high demand for proline, an essential amino acid component of collagen. Deficiency in the mitochondrial NAD⁺ pool, regulated by NAD⁺ salvage and transport, hinders proline biosynthesis in mitochondria, contributing to thoracic and abdominal aortic aneurysm.