BACKGROUND: School-based extracurricular sport remains an effective strategy to increase physical activity. However, school sport is often limited to a small number of elite athletes. Few schools provide more inclusive sport programs that offer a wider array of activities regardless of ability. PURPOSE: The aim of this study was to examine school sport participation in middle schools (ages 11–14) with contrasting school sport delivery strategies (intramural vs. interscholastic). METHODS: Data were obtained through an online survey administered to students at four public middle schools (grades 6–8) in a southeastern US city (n = 2,582). RESULTS: More students participated in school sports at intramural schools. Boys were more likely to participate in after-school sports at intramural schools. Low-income and Black children, two groups at greater risk of physical inactivity and other negative outcomes, had greater participation in intramural programs. CONCLUSIONS: After-school intramural sports in middle school is a promising strategy for increasing sport participation.