Attention-deficit hyperactivity disorder (ADHD) is a developmental disorder categorized by symptoms of inattention, impulsivity, and/or hyperactivity. The onset of ADHD symptoms begin to appear in childhood, tend to be persistent into adulthood, and are associated with adverse physical and mental health outcomes (Caye et al., 2016; Yoshimasu et al., 2018). The current study examined the effects of health-promoting behavior (i.e., diet and physical activity) on ADHD symptoms in middle childhood. The sample consisted of 670 twins (30.1% monozygotic, 36.5% same-sex dizygotic, 32.3% opposite-sex dizygotic) from the ongoing longitudinal study, the Arizona Twin Project. The twin’s (51% female) diet, physical activity, and ADHD symptoms were assessed at age 8 (M = 8.45, SD = 0.69) and ADHD symptoms were assessed again at age 9 (M = 9.70, SD = 0.92). Mixed model regression analyses revealed that aspects of diet (i.e., protein, carbohydrates, and fiber) at age 8 negatively predicted ADHD symptoms at age 9. Similarly, sedentary behavior at age 8 negatively predicted ADHD symptoms at age 9, whereas moderate-to-vigorous activity at age 8 positively predicted ADHD symptoms at age 9. Univariate twin analyses revealed that certain aspects of diet (i.e., sugar, vegetable, and fruit consumption) were influenced by environmental factors whereas other aspects of diet (i.e., protein, carbohydrates, calories, fat, and fiber consumption) were influenced by both genetic and environmental factors. Both children’s sedentary behavior and moderate-to-vigorous activity were influenced primarily by genetic factors, with the remaining variance being attributed to non-shared environmental factors. Additive genetic influences explained the majority of the variance in ADHD symptoms. Future research should examine bidirectional effects of activity and diet on ADHD symptoms across childhood.