In line with the current psychological approach to health in general, mental health is perceived not only as the absence of psychopathological disorders, but also the presence of well-being. The study contributes to the identification of possible sources affecting mental health in adolescence. This cross-sectional study focuses on the role of personality traits, dispositional optimism, and perceived social support in predicting mental health in adolescence. Mental health was assessed using Mental Health Continuum, personality traits using Big Five Inventory, dispositional optimism using Life Orientation Test—Revised and social support by Close Relationships and Social Support Scale. The research sample consisted of 1,239 respondents aged 12 to 19 years (mean age 15.56 years), 54.3% females and 45.7 % males. Sequential regression analysis revealed that demographic variables and personality characteristics together explained 33.5% of mental health variance, the strongest predictors being extraversion and neuroticism. Including dispositional optimism and perceived social support resulted in a significant increase of the explained variance. All predictors together explained 46.0% of the mental health variance.