Background: Motivating adolescents to maintain levels of physical activity (PA) is important because regular PA in adolescence contributes to physical, psychological, and social well-being and PA during adolescence has been associated with activity levels in adulthood. Purpose: The overall aim of this study is to validate a measure of external reward, health values, and personal interest motivations for adolescent PA developed by Wold and Kannas and to examine the relationship between these motivations and level of PA. Methods: A nationally representative sample of 9011 adolescents completed the Health Behavior in School-aged Children survey instrument. Ten items were used to measure PA motivations. Multiple group confirmatory factor analysis and structural equation modeling were applied to test the 3-factor structure of the motivation scale and to examine the relationship between the 3 motivations and PA. Results: The Wold and Kannas's motivation measure assessed external, social, and health motivations which predicted PA in adolescents. Conclusions: The Wold and Kannas's motivation measure is suitable for assessing motivations for PA in US adolescents and may contribute to both theoretical and intervention studies that address this public health need.