Objective: The purpose of this study was to identify the psychosocial determinants of (1) intention to use dietary supplements (DS) and (2) the behavior of engaging in supplementation by testing the fit of the Theory of Planned Behavior (TPB). Methods: A total of 778 students at a Canadian university in Ontario completed a cross-sectional survey assessing participants' attitude, injunctive norm, descriptive norm, perceived behavioral control and intention toward DS use. Data were analyzed using independent sample t-tests and linear and logistic regressions. Results: Attitude, injunctive norm and perceived behavioral control were significant predictors of intention to use DS. Attitude, injunctive norm and intention were significant predictors of engaging in supplementation. The model explained 75.5% of the variance and correctly classified 89.6% of cases. The odds of using DS doubled with every one-unit increase in intention. Conclusions: This study confirms the utility of the TPB in predicting university students' supplementation habits.