Purpose: This research describes the development of a measure of the general attribute of "athletic" in adolescents, encompassing exercise, sport, and physical activity. Based on a theoretical model supported in adults, the 40-item Athletic Identity Questionnaire (AIQ) for adolescents assesses four dimensions: appearance, competence, importance of activity, and encouragement from three sources: parents, friends, and teachers/ other adults. Methods: Structural equation modeling was used to evaluate the hypothesized four-factor model in a development sample of 408 adolescents in eighth grade (mean age 13.4 yr). A separate sample (N = 1586) was used to cross-validate the final model. Construct validity was examined by testing the model's relationship to self-reported (Modifiable Activity Questionnaire-Adolescent, Previous Day Physical Activity Recall, Youth Risk Behavior Survey) and objectively measured physical activity (MTI accelerometer in sample 3, N = 100). Results: Confirmatory factor analysis supported the four-factor structure, and there was also support for a higher-order model. LISREL correlations between the AIQ factors and self-reported physical activity ranged from 0.32 to 0.61, TV watching from -0.20 to -0.50, and sport-team participation from 0.20 to 0.54. Pearson correlations between the AIQ factors and MTI vigorous physical activity ranged from 0.09 to 0.26 and MTI moderate from -0.06 to 0.22. Conclusions: Findings support the factorial and construct validity of the AIQ for adolescents.