Objectives: We examined the longitudinal relationship between cardiovascular fitness (CRF) and academic performance (AP) among students in a diverse public school district. Furthermore, we determined whether the relationship between CRF and AP varied by student sociodemographic characteristics. Methods: This study used data from 2005-2006 to 2008-2009. AP was assessed by Massachusetts Comprehensive Assessment System (MCAS) math (N = 1224) and English Language Arts (ELA) (N = 1248) z-scores. CRF was assessed by an age- and sex-adjusted 20-yard shuttle run (pass/fail). Multilevel growth models were used to estimate the relationship between MCAS z-scores and changes in grade, CRF, and weight status, controlling for student sociodemographic characteristics. Results: Students who passed the fitness test performed better on MCAS tests (ELA: 0.159, p < 0.001; Math: 0.346, p < 0.001) at each study wave relative to those who did not pass. The relationship between AP and fitness was attenuated for low socioeconomic status (ELA: -0.111, p < 0.05) and minority (Math: black -0.249, p < 0.01; Latino -0.216, p < 0.01) students. Conclusions: These results add to the literature highlighting the role of CRF as an important predictor of performance on standardized tests. More research is needed to understand the disparities experienced by low-income and minority students.