Background: Mental health problems in adolescents have become a major public health challenge in many western countries. Many adolescents participate in noncompetitive or competitive sports for leisure. The aim of this study was to examine the association between participation in noncompetitive or competitive sports and mental health problems among Norwegian adolescents. Method: Data from a cross-sectional survey conducted among Norwegian junior high (N=9414) and high school students (N=10,571) aged 13 to 22 years were examined. The Strengths and Difficulties Questionnaire (SDQ) was used for measuring mental health problems, in the form of total symptom scores and impact on daily life. Participation in sports was categorized as follows: 1) not participating; 2) participating in noncompetitive sports; and 3) participating in competitive sports. Sports were divided into individual and team sports. Weekly exercise, healthy diet, and number of friends were investigated as mediating factors, and age, gender, and perceived economic status were controlled for in the analysis. Results: Adolescents participating in either noncompetitive or competitive sports had fewer total symptoms and less impact of problems on their daily life compared with those not participating in sports. In particular, participation in team sports as opposed to participation in individual sports seemed to be associated with fewer mental health problems. Weekly exercise, healthy diet, and number of friends were found to be partially mediating factors, explaining some, but not all, of the association between participation in either sports group. Participation in competitive sports, especially, was associated with fewer mental health problems. Conclusions: Participation in sports, either noncompetitive or competitive, is associated with fewer mental health problems in adolescents. This negative association is strongest for those participating in competitive sports. Further research is needed to explore the mechanisms underlying this relationship.