Background and Objective: Increased emphasis on academic outcomes has reduced the amount of time spent in physical education and other school physical activity opportunities in many schools in the USA. However, physical fitness is a positive predictor of academic performance on standardised tests, and students who perform better on fitness measures may earn higher grades. The purpose of this study was to evaluate the relationship between physical fitness and teacher-assigned grades in fourth-grade students and examine whether the relationship is moderated by body composition, gender or ethnic background. Design: Cross-sectional design. Students' fitness levels were assessed mid-way through the spring semester, and their third-quarter grades were obtained from the schools. Participants and setting: Fourth-grade students (N = 80; 38 female students; 65 non-Hispanic or Latino) from two elementary schools in the south-western USA. Method: Students completed physical fitness measures using standard procedures from the "FITNESSGRAM" protocol, and standardised teacher-assigned grades in reading, writing, mathematics, social studies and science were compiled. Linear regression and multivariate analysis of variance (MANOVA) were performed to examine the relationship between physical fitness measures and average grades, and examine whether differences in fitness and grades existed between gender, ethnic background and body mass index (BMI) percentile rank. Results: Aerobic fitness, as measured by the number of 20-m Progressive Aerobic Cardiovascular Endurance Run (PACER) laps completed, had a significant influence on reading, writing, mathematics and science grades. There were no significant interactions between aerobic fitness and ethnic background, gender or BMI percentile rank, meaning that aerobic fitness was the largest specific influence on average teacher-assigned grades. Conclusion: Positive associations exist between physical fitness and academic performance, suggesting activities that help children improve their physical fitness ought to be a central component of physical education and broader school physical activity programmes.