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Today's young people (aged ∼10-24 y) are the first "digital natives" but have become the most sedentary generation on record. These trends have coincided with a widely recognized decline in youth mental health, prompting increasingly polarized debate: some attribute the crisis primarily to smartphones and social media, but large-scale analyses suggest more heterogeneous population-level effects. Therefore, this review examines the role of physical activity (PA) in mental health during the online era; considering how different forms of online engagement relate to youth mental health, with regard to both the emerging downsides (through increasing sedentariness) and the potential for harnessing such platforms to improve PA and mental health, together. As a narrative review of an emerging field, we drew on nonsystematic searches of PubMed and Google Scholar alongside author expertise to integrate key findings and identify new directions. Two conclusions emerged. First, whether online time helps or harms depends more on content, context, and user characteristics than screentime, although converging (though still-developing) causal evidence points to displacement of PA, alongside disrupted sleep as a leading mechanism. Second, digital adoption itself has become an opportunity for promoting PA/delivering exercise interventions, online, with growing evidence of benefits for depression, anxiety, and well-being. However, these tools also carry risks, including body surveillance, social comparison, and compulsive exercise. Furthermore, data on online PA interventions in clinical mental health populations is sparse but appear to favor blended approaches. Overall, we argue the coming decade's key challenge is embedding evidence-based PA promotion within young people's online lives.