Importance: Race-specific and sex-specific stroke risk varies across the lifespan, yet few reports describe sex differences in stroke risk separately in black individuals and white individuals. Objective: To examine incidence and risk factors for ischemic stroke by sex for black and white individuals. Design, Setting, and Participants: This prospective cohort study included participants 45 years and older who were stroke-free from the Reasons for Geographic and Racial Differences in Stroke (REGARDS) cohort, enrolled from the continental United States 2003 through 2007 with follow-up through October 2016. Data were analyzed from March 2018 to September 2018. Exposures: Sex and race. Main Outcomes and Measures: Physician-adjudicated incident ischemic stroke, self-reported race/ethnicity, and measured and self-reported risk factors. Results: A total of 25 789 participants (14 170 women [54.9%]; 10 301 black individuals [39.9%]) were included. Over 222 120 person-years of follow-up, 939 ischemic strokes occurred: 159 (16.9%) in black men, 326 in white men (34.7%), 217 in black women (23.1%), and 237 in white women (25.2%). Between 45 and 64 years of age, white women had 32% lower stroke risk than white men (incidence rate ratio [IRR], 0.68 [95% CI, 0.49-0.94]), and black women had a 28% lower risk than black men (IRR, 0.72 [95% CI, 0.52-0.99]). Lower stroke risk in women than men persisted at age 65 through 74 years in white individuals (IRR, 0.71 [95% CI, 0.55-0.94]) but not in black individuals (IRR, 0.94 [95% CI, 0.68-1.30]); however, the race-sex interaction was not significant. At 75 years and older, there was no sex difference in stroke risk for either race. For white individuals, associations of systolic blood pressure (women: hazard ratio [HR], 1.13 [95% CI, 1.05-1.22]; men: 1.04 [95% CI, 0.97-1.11]; P = .099), diabetes (women: HR, 1.84 [95% CI, 1.35-2.52]; men: 1.13 [95% CI, 0.86-1.49]; P = .02), and heart disease (women: HR, 1.76 [95% CI, 1.30-2.39]; men, 1.26 [95% CI, 0.99-1.60]; P = .09) with stroke risk were larger for women than men, while antihypertensive medication use had a smaller association in women than men (women: HR, 1.17 [95% CI, 0.89-1.54]; men: 1.61 [95% CI, 1.29-2.03]; P = .08). In black individuals, there was no evidence of a sex difference for any risk factors. Conclusions and Relevance: For both races, at age 45 through 64 years, women were at lower stroke risk than men, and there was no sex difference at 75 years or older; however, the sex difference pattern may differ by race from age 65 through 74 years. The association of risk factors on stroke risk differed by race-sex groups. While the need for primordial prevention, optimal management, and control of risk factors is universal across all age, racial/ethnic, and sex groups, some demographic subgroups may require earlier and more aggressive strategies.