Children born soon after the selection period’s cut-off date (early-born children) may benefit by up to a full year in physical and psychological development compared with children born just before the cut-off date (late-born children). This phenomenon is called the "relative age effect" (RAE). RAE is defined as the consequence of age differences between individuals within the same cohort, either in school or on sports teams. We examined the RAE on physical activity enjoyment, physical activity competence, attitude toward physical activities, attitude toward physical education, and frequency of participation in physical activities among Japanese elementary school children. Data were collected from 961 Japanese elementary school children (478 male, 483 female). The results showed that early-born children scored higher than late-born children did in the above-mentioned variables. This indicated that the RAE exists for psychological factors related to sports participation among Japanese elementary school children.