Background Most children and young people (CYP) worldwide are classified as inactive because they fail to meet the World Health Organisation recommendations for physical activity. Online interventions that use devices like exergames, smartphones, social media, and wearables have the potential to improve physical activity engagement because of their extensive reach and opportunities for learning and use across contexts. Purpose The objectives of this systematic review were to update the evidence-base on online physical activity interventions for CYP since 2015, analyse the outcomes associated with online interventions across physical, cognitive, social and affective domains, and assess the mechanisms (i.e. pedagogical strategies) of online interventions that resulted in outcomes related to physical activity. Methods A systematic search of the literature was conducted across 4 databases (MEDLINE, PudMed, EBSCO and EMBASE) using key words related to online interventions, physical activity and CYP. The inclusion criteria were: CYP aged 5-18 years in the general population; use of an online-based medium to deliver an intervention related to physical activity; outcomes related to changes to physical activity, and in physical, cognitive, social and affective domains; and quantitative, qualitative and mixed methods studies. A modified version of the Quality Assessment Tool for Studies with Diverse Designs was used to assess study quality. A mixed methods approach was used to analyse and synthesise all evidence. Results 26 papers were identified as meeting the inclusion criteria, including randomised control trials (n=8), non-randomised interventions (n=12), observational studies (n=3) and qualitative papers (n=3). The target population of most studies was children (<12 years) where data collection mostly took place in a school setting, in elementary schools, and in physical education lessons. The interventions reported on positive changes to CYP's physical activity behaviours, through increases in physical activity levels and emotions, attitudes and motivations toward physical activity. Gamification and personalisation were the main mechanisms of online interventions that elicited positive changes in behaviours. Conclusions The studies in this review provide a convincing rationale for the use of online interventions to support CYP's engagement with physical activity, due to the positive effects on physical and affective outcomes. New evidence is provided on the key mechanisms of online interventions (gamification and personalised learning) and the contexts in which online interventions are likely to be effective (elementary school PE) that can be used by health and education practitioners, organisations, policy makers and/or researchers to reach and engage CYP in physical activity. This study had some limitations that mainly relate to variation in study design, over-reliance of self-reported measures, and sample characteristics, that prevented comparative analysis. Registration number: PROPSERO; CRD42020215597.