Background Recent studies have shown an increasing prevalence of vascular risk factors in young adults with ischemic stroke ( IS ). However, the strength of the association between all vascular risk factors and early‐onset IS has not been fully established. Methods and Results We compared 961 patients with a first‐ever IS at 25 to 49 years to 1403 frequency‐matched stroke‐free controls from a population‐based cohort study ( FINRISK ). Assessed risk factors included an active malignancy, atrial fibrillation, cardiovascular disease, current smoking status, a family history of stroke, high low‐density lipoprotein cholesterol, high triglycerides, low high‐density lipoprotein cholesterol, hypertension, and type 1 and type 2 diabetes mellitus. We performed subgroup analyses based on age, sex, and IS etiology. In a fully adjusted multivariable logistic regression analysis, significant risk factors for IS consisted of atrial fibrillation (odds ratio [OR], 10.43; 95% confidence interval [ CI ], 2.33–46.77], cardiovascular disease (OR, 8.01; 95% CI , 3.09–20.78), type 1 diabetes mellitus (OR, 6.72; 95% CI , 3.15–14.33), type 2 diabetes mellitus (OR, 2.31; 95% CI , 1.35–3.95), low high‐density lipoprotein cholesterol (OR, 1.81; 95% CI , 1.37–2.40), current smoking status (OR, 1.81; 95% CI , 1.50–2.17), hypertension (OR, 1.43; 95% CI , 1.17–1.75), and a family history of stroke (OR, 1.37; 95% CI , 1.04–1.82). High low‐density lipoprotein cholesterol exhibited an inverse association with IS . In the subgroup analyses, the most consistent associations appeared for current smoking status and type 1 diabetes mellitus. Conclusions Our study establishes the associations between 11 vascular risk factors and early‐onset IS , among which atrial fibrillation, cardiovascular disease, and both type 1 and 2 diabetes mellitus in particular showed strong associations.