Background: Socioeconomic status (SES) is the most accurate predictor of academic performance in US schools. Third-grade reading is highly predictive of high school graduation. Chronic physical activity (PA) is shown to improve cognition and academic performance. We hypothesized that school-based PA opportunities (recess and physical education) would moderate the negative association between SES and third-grade reading. Methods: Schools serving third-grade students were surveyed (N = 1279) for minutes/week of PA opportunities. Allotted weekly PA time and achievement data from participating schools (N = 784) were recorded and analyzed. To test the moderator hypothesis, moderated multiple regression analysis was conducted. Results: The interaction of PA opportunities and SES explained a significant increase in variance in third-grade reading (b = 0.053, p < 0.001), thus moderating the relationship between SES and third-grade reading. Further analysis showed that schools offering greater than 225 minutes/week of PA opportunities experienced a greater (+5%) moderating effect. Conclusion: School-based PA opportunities positively moderate the relationship between SES and third-grade reading, and lowest SES schools experience greater moderating effects. Future research should consider PA opportunities as a moderator of the SES-academic achievement relationship, and school policy makers should consider the influence that PA opportunities have on student achievement at varying SES levels.