Objective We aimed to study the association between neighborhood socioeconomic status at the age of 40 years and risk of ischemic stroke before the age of 50 years. Methods All individuals in Sweden were included if their 40th birthday occurred between 1998 and 2010. National registers were used to categorize neighborhood socioeconomic status into high, middle, and low and to retrieve information on incident ischemic strokes. Hazard ratios and their 95% confidence intervals were estimated. Results A total of 1,153,451 adults (women 48.9%) were followed for a mean of 5.5 years (SD 3.5 years), during which 1777 (0.30%) strokes among men and 1374 (0.24%) strokes among women were recorded. After adjustment for sex, marital status, education level, immigrant status, region of residence, and neighborhood services, there was a lower risk of stroke in residents from high-socioeconomic status neighborhoods (hazard ratio 0.87, 95% confidence interval 0.78–0.96), and an increased risk of stroke in adults from low-socioeconomic status neighborhoods (hazard ratio 1.16, 95% confidence interval 1.06–1.27), compared to their counterparts living in middle-socioeconomic status neighborhoods. After further adjustment for hospital diagnoses of hypertension, diabetes, heart failure, and atrial fibrillation prior to the age of 40, the higher risk in neighborhoods with low socioeconomic status was attenuated, but remained significant (hazard ratio 1.12, 95% confidence interval 1.02–1.23). Conclusions In a nationwide study of individuals between 40 and 50 years, we found that the risk of ischemic stroke differed depending on neighborhood socioeconomic status, which calls for increased efforts to prevent cardiovascular diseases in low socioeconomic status neighborhoods.