question: Background: The COVID-19 pandemic has exacerbated the already existing pandemic of physical inactivity. In adolescents, the effect of COVID-19related governmental measures may beeven more pronounced,as they experience school closures, home confinement, social distancing and possible mental health issues as a result.The currentstudy uses the COVID-19 pandemic and associated governmental measures as a natural experiment to understand the impact it may have on the lifestyle of adolescents with relation to physical activity, sedentary and dietary behaviour. We also aim to describe the determinants of change in lifestyle behaviours. Methods: /Design: A before-after experimental study design will be followed in which baseline measurements have been undertaken in 2018-19 before the COVID-19 pandemic, and the follow-up measurements will be repeated during the COVID-19 pandemic. Twenty government and private schools of Punjab, India form the primary unit of participation from which 1086adolescents participated in the study. Validated questionnaires are used to measurephysical activity, sedentarybehaviour, and dietary behaviour. Additionally, objective measures of physical activity will be performed using ActiGraph wGT3X-BT over a 7-day period on a sub-sample. Body mass index (BMI) and waist circumference will be assessed objectively as a measure of central obesity. Qualitative interviews and focus group discussions will be conducted to have a better understanding for reasons of change in physical activity, sedentary and dietary behaviour or any possible differences between groups. Discussion: The study aims torevealthe impact of COVID-19 governmental measures on physical activity, sedentary and dietary behaviour of adolescents. It is one of the first studies to show both subjective and objective measures of physical activity levels and sedentary time of school-going adolescents during COVID-19. Due to the mixed method approach we will be able to identify reasons for changes in lifestyle behaviours and possibly also detect innovative ways of stayingphysically active during the era of physical distancing.