Using structural equation modeling, this study found that prior reading skill was significantly associated with children's reported physical activity levels, which was in turn significantly associated with English language arts (ELA) and math achievement, among 526 3rd-grade students. The New York City Metropolitan area students completed the Physical Activity Questionnaire for Older Children (PAQ-C), which measured average activity levels. Controlling for race, ethnicity, Body Mass Index, learning disability status, and socioeconomic status at the child and school levels, developmental reading assessment (DRA) scores had a strong, independent association with physical activity levels, which in turn had a moderate association with current ELA and a strong association with math achievement. Hispanic ethnicity had a significant negative association with activity levels (relative to Whites), though the positive association of physical activity with ELA and math achievement was stronger among Hispanics than it was among the samples of Black and White students. These findings suggest that enhanced academic achievement significantly influences, and is significantly influenced by, student physical activity levels. Schools should consider ways to increase student physical activity, including using low/no cost in-class exercises.