Both the transdiagnostic risk associated with depression and the eating disorder-specific risk associated with expectancies for reinforcement from thinness have been identified as risk factors for the development of weight restricting behaviors. The purpose of this study was to examine if these risk factors transact to further predict risk in youth. Depression, thinness expectancies, and weight restriction were assessed in 1,907 adolescents three times during the transitional period between middle school and high school. We compared three different possible transactional processes. Mediation tests demonstrated that depression in 8th grade predicted an increase in the number of weight restricting behaviors endorsed in 10th grade through its predictive influence on thinness expectancies in 9th grade. However, our results were not consistent with a mediational process in which thinness expectancies predicted depression to further predict weight restriction. The two risk factors interacted to predict subsequent weight restriction, such that at higher levels of depression, the association between thinness expectancies and weight restriction was stronger. It appears that transdiagnostic and disorder-specific risk factors transact to increase risk. These findings contribute to the understanding of the developmental risk process for weight restricting behavior in youth.