Background: As youth struggle with anxiety and depression, promoting positive mental fitness is a primary concern. Canadian school-based mental health programs that focus on positive psychology and positive mental health initiatives emphasize safe and supportive environments, student engagement, resilience, and self-determination. This study examined predictors of mental fitness and its 3 components (autonomy, competence, and relatedness). Methods: School Health Action Planning and Evaluation System-Prince Edward Island (SHAPES-PEI) and the New Brunswick Student Wellness Survey (NB SWS) are data collection and feedback systems that survey youth about 4 health behaviors. Grade 7-12 students in Prince Edward Island (N?=?3318) and New Brunswick (N?=?7314) completed a mental fitness questionnaire in 2008-2009 (PEI) and 2006-2007 (NB). Four linear regression models were conducted to examine student characteristics associated with mental fitness, autonomy, competence, and relatedness. Results: Positive associations were found between school connectedness (p?<?0.0001) and mental fitness, as well as autonomy, competence, and relatedness. There were also significant relationships between affect, pro-social and antisocial behaviors, tried smoking, and mental fitness. Conclusion: A better understanding of adolescent health and its predictors is needed. By identifying core parameters for mental fitness, we can inform how to address students' needs through appropriate programs and policies supporting healthy school environments.