Introduction:The World Health Organisation recommends that children and adolescents engage in at least 60 min of moderate-to-vigorous physical activity per day. Previous research has shown that physical activity is related to other constructs such as mental well-being and self-rated health. This study examined the interrelatedness of these constructs in Northern Irish school children. Methods: This study was a secondary analysis of data gathered as part of a longitudinal study. Participants were n = 1791 adolescents in their final years of secondary (high) school (age range 15-18; female = 64.6%). Data were gathered on three occasions over a 2-year period on self-rated health, physical activity, mental well-being, heavy episodic drinking, lifetime smoking, psychological and somatic symptoms, as well as a range of socio-demographic measures. Results: Descriptive results showed extremely low levels of self-reported physical activity within the past week, with <6% of the sample attaining the WHO guidelines at each wave of data collection. There were significant gender differences on all variables assessed. Results further showed a small-sized relationship (statistically significant for girls only) between physical activity and mental well-being. There was also a smallsized relationship between physical activity and self-rated health. Notably, effect sizes for the relationship between self-rated health and both physical activity and mental well-being were higher. In terms of socio-demographic predictors of lower physical activity, being female, lifetime cigarette smoking, and higher somatic and psychological symptoms were all statistically significant factors. Conclusion: Self-rated health emerged as the most important predictor of physical activity among adolescents.physical activity, self-rated health, mental well-being, secondary school age Physical activity (PA) plays a critical role in the normal growth and healthy development of young people, and can be a predictor of their continued well-being through to adulthood (Tremblay et al., 2012). The World Health Organisation (WHO) has recommended that children ought to engage in a minimum of 60 min of moderate-to-vigorous physical activity (MVPA) per day to maintain health benefits (WHO, 2010). It is therefore of concern to note recent findings from the Health Behaviour in School-aged Children study, which reveal that globally, fewer than one in five adolescents achieve the daily recommended levels of MVPA (Inchley et al., 2020).