Background Dietary intake and blood concentrations of vitamins E and C, lycopene, and carotenoids have been associated with a lower risk of incident (ischemic) stroke. However, causality cannot be inferred from these associations. Here, we investigated causality by analyzing the associations between genetically influenced antioxidant levels in blood and ischemic stroke using Mendelian randomization. Methods and Results For each circulating antioxidant (vitamins E and C, lycopene, β-carotene, and retinol), which were assessed as either absolute blood levels and/or high-throughput metabolite levels, independent genetic instrumental variables were selected from earlier genome-wide association studies (<5×10). We used summary statistics for single-nucleotide polymorphisms-stroke associations from 3 European-ancestry cohorts (cases/controls): MEGASTROKE (60 341/454 450), UK Biobank (2404/368 771), and the FinnGen study (8046/164 286). Mendelian randomization analyses were performed on each exposure per outcome cohort using inverse variance-weighted analyses and subsequently meta-analyzed. In a combined sample of 1 058 298 individuals (70 791 cases), none of the genetically influenced absolute antioxidants or antioxidant metabolite concentrations were causally associated with a lower risk of ischemic stroke. For absolute antioxidants levels, the odds ratios (ORs) ranged between 0.94 (95% CI, 0.85-1.05) for vitamin C and 1.04 (95% CI, 0.99-1.08) for lycopene. For metabolites, ORs ranged between 1.01 (95% CI, 0.98-1.03) for retinol and 1.12 (95% CI, 0.88-1.42) for vitamin E. Conclusions This study did not provide evidence for a causal association between dietary-derived antioxidant levels and ischemic stroke. Therefore, antioxidant supplements to increase circulating levels are unlikely to be of clinical benefit to prevent ischemic stroke.