The purpose of this study is to investigate associations between physical activity (PA), depression, and anxiety among adolescents in Ireland. Adolescents (N = 481; 281 male, 200 female) aged 15.1 +/- 1.7y self-reported PA level, depression, and anxiety. Approximately 21% of adolescents were high trait anxious, and similar to 37% reported scores indicating probable depression. Anxiety and depressive symptoms were higher for low PA (60 min/d, 0-2 d/wk) compared to moderate (60 min/d, 3-4 d/wk) and high (60 min/d, 5-7 d/wk) PA. After adjustment for relevant covariates, reduced odds of depression were 30% and 56% for moderate and high PA, respectively; reduced odds of high trait anxiety were 46% and 47% for moderate and high PA, respectively. These findings support the need for adolescents to engage in moderate PA, with potential for increased benefits with increased PA. To conclude, moderate and high PA are inversely associated with anxiety and depressive symptoms, and risk of depression and high trait anxiety in adolescents. (C) 2016 The Foundation for Professionals in Services for Adolescents. Published by Elsevier Ltd. All rights reserved.