Thoracic aortic aneurysm (TAA) predisposes to sudden, life-threatening aortic dissection. The factors that regulate interindividual variability in TAA severity are not well understood. Identifying a molecular basis for this variability has the potential to improve clinical risk stratification and advance mechanistic insight. We previously identified COQ8B, a gene important for biosynthesis of coenzyme Q, as a candidate genetic modifier of TAA severity. Here, we investigated the physiological role of COQ8B in human aortic smooth muscle cells (SMCs) and further tested its genetic association with TAA severity. We find COQ8B protein localizes to mitochondria in SMCs, and loss of mitochondrial COQ8B leads to increased oxidative stress, decreased mitochondrial respiration, and altered expression of SMC contractile genes. Oxidative stress and mitochondrial cristae defects were prevalent in the medial layer of human proximal aortic tissues in patients with TAA, and COQ8B expression was decreased in TAA SMCs compared with controls. A common single nucleotide polymorphism (SNP) rs3865452 in COQ8B (c.521A>G, p.H174R) was associated with decreased rate of aortic root dilation in young patients with TAA. In addition, the SNP was less frequent in a second cohort of early-onset thoracic aortic dissection cases compared with controls. COQ8B protein levels in aortic SMCs were increased in TAA patients homozygous for rs3865452 compared with those homozygous for the reference allele. Thus, COQ8B is important for aortic SMC metabolism, which is dysregulated in TAA, and rs3865452 may decrease TAA severity by increasing COQ8B level. Genotyping rs3865452 may be useful for clinical risk stratification and tailored aortopathy management.