The purpose of the study was to explore change in children's physical self-concept and self-reported physical activity over a school transition period, as well as motivational and interpersonal influences on these two outcomes. Data were collected from 545 children (mean age = 10.82, SD = 0.39, 51% female) at three time points before and after the United Kingdom secondary school transition. Multilevel modeling revealed that physical self-concept and physical activity showed different patterns of decline over the course of the study. Changes in the extent to which physical education teachers were perceived to provide psychological need support, peer focus on self-referenced learning and mastery, and changes in autonomous motives toward physical education classes were positively associated with these outcome variables. The present study provides novel insight into important motivational and interpersonal factors that may need to be targeted to prevent negative developmental patterns over a potentially challenging period for children.