Adolescents often engage in behaviours (poor diet, physical inactivity, sedentary behaviour, poor mental health, inadequate sleep) that put them at risk of developing chronic diseases such as cardiovascular disease. Smartphone applications (apps) are widely used by adolescents and have become an integral part of their lives therefore, we aimed to investigate the quality of health apps targeting adolescents. A systematic search of the Apple iTunes and Google Play stores was conducted between January-March 2020. Apps were included if they: targeted adolescents as specified in the description, were in English, had a user rating of 3.5+, targeted one of the following health behaviours: diet, physical activity, sedentary behaviour, mental health or sleep. Apps were downloaded and data was extracted using a predefined set of features and reviewed using the Mobile App Rating Scale (MARS). 10,998 apps were identified, of these 56 apps were further screened by description. Apps were excluded due to not targeting adolescents (n=23), no or low user rating (n=16) or no longer available (n=2). 16 were downloaded for testing and 9 were included in the review. N=4 targeted physical activity and n=5 targeted mental health. The average user rating of the apps was 4.4, although most had a small number of ratings (<20), indicating that they are not very popular. On a 5-point MARS, overall app quality was moderate (3.8/5). There are limited mobile apps designed specifically for adolescents and their overall quality is average. App development should involve a co-design process to ensure that apps are high-quality, relevant and engaging to this age group.