The health benefits of physical activity are empirically supported and well accepted. However, the relationship between physical activity, physical fitness, and academic performance remains to be clearly established. The purpose of this study was to identify the relationship between physical fitness and academic achievement among a sample of Illinois high school students. Analyses were based on the 2016-2017 school year Archival Fitnessgram physical fitness test scores and cumulative GPAs of ninth- through twelfth-grade students (N = 371). Pearson correlation assessed the relationship between physical fitness and academic performance. Multiple linear regression predicted students' academic achievement. There was a positive association between total fitness and academic achievement, although not statistically significant, r (369) = 0.002, p = 0.49. The regression prediction model was statistically significant (p < 0.001) and accounted for approximately 25% of the variance in academic achievement (R[superscript 2] = 0.256, adjusted R[superscript 2] = 0.246). Academic achievement was predicted by total number of absences and gender, and to a lesser extent by socio­economic status, the curl-up, and ethnicity. The findings of this study suggest a positive association between physical fitness and academic achievement. These results are potentially relevant to the development of future education policies. Thus, policy makers, school administrators, and educators must use the knowledge gained in this study, along with existing research, as evidence to emphasize the importance of the fitness--academic link, to further support the need for quality physical education curriculum and mandated physical fitness testing.