Background: This study examined the association between physical fitness and academic achievement and determined the influence of socioeconomic status (SES) on the association between fitness and academic achievement in school-aged youth. Methods: Overall, 1,701 third-, sixth-, and ninth-grade students from 5 school districts participated in the assessments. Fitness was assessed using FITNESSGRAM (aerobic fitness, muscular strength, muscular endurance, flexibility, and body composition). Results were used to determine individual fitness scores. Academic achievement was measured by standardized tests for Math (all grades), English (all grades), and Social Studies (sixth and ninth grades only). The SES was determined using eligibility for free and reduced lunch program. Results: There were no significant differences between fitness groups for Math and English in third-grade students. Sixth- and ninth-grade students with high fitness scored significantly better on Math and Social Studies tests compared with less fit students. Lower SES students scored significantly worse on all tests. Muscular strength and muscular endurance were significantly associated with academic achievement in all grades. Conclusions: Compared with all other variables, SES appears to have the strongest association with academic achievement. However, it also appears that high fitness levels are positively associated with academic achievement in school-aged youth. (Contains 4 tables and 1 figure.)