Background Physical inactivity and sedentary behaviour are the main risk factors for non-communicable diseases in all stages of life. However, there is a lack of reliable data regarding the recommended level of physical activity and its correlates related to physical inactivity and sedentary behaviour among school students in Iran. The aim of the present study was to report the prevalence and correlates of physical activity/ inactivity and sedentary behaviour among Iranian high school adolescents.Methods The Global School-based Student Health Survey (GSHS) was used for data collection. Related data comprising socio-demographics, health risk behaviour, and protective factors were obtained from 1,517 high school students. Multivariate logistic regression was used to assess the association between relevant independent variables (e.g. gender) and the dependent outcome variable (physical inactivity/sedentary behaviour).Results The prevalence of recommended physical activity by WHO was 27.8%, physical inactivity 72.2% and sedentary behaviour 71.4%. Being female, being overweight or obese, walking/cycling to or from school on 1-4 days, sitting 3 or more hours/day (sedentary behaviour), insufficient vegetable intake, being bullied, and lack of parental support, peer support and parental connectivity (protective factors) were positively associated with physical inactivity. On the other hand, walking/cycling to or from school on 5-7 days, and inadequate fruit intake was negatively associated with physical inactivity. Walking/cycling to or from school on 1-4 days, being physically inactive, inadequate fruit intake, and being bullied were positively associated with sedentary behaviour.Conclusion The prevalence of physical inactivity and sedentary behaviour was high in our studied sample. Interventions that address the issue of active school transport, sedentary behaviour, fruit and vegetable intake, family and peer support and bullying should be given more priority by the public health authorities.