<p>Self-harm has proven social contagion effects among young people. However, a comprehensive understanding of the complex dynamics that contribute to self-harm contagion in adolescents is lacking. In this Review, we synthesize evidence regarding the social contagion of self-harm in young people using a social ecological approach. At the individual level, psychological and neurobiological vulnerabilities increase young people’s susceptibility to social contagion. At the interpersonal level, social contagion of self-harm occurs through peer interactions and social media connections, as well as through family ties including parental, sibling and grandparental relationships. At the community level, social contagion is evident in high-risk clusters of young people in institutional settings (schools, universities, psychiatric hospitals and justice-involved youth institutes), on social media and in digital spaces, and in neighbourhoods, where socioeconomic disadvantage is a key structural constraint that amplifies self-harm contagion. At the societal level, media-regulation challenges, global pandemics and political context exacerbate the social contagion of self-harm by intensifying pre-existing risk factors across individual, interpersonal and community levels. We address these multilevel factors to bridge psychological and public health perspectives of social contagion dynamics and describe prevention and intervention efforts that might offer scalable, evidence-based solutions for mitigating self-harm among youth.</p>