The Social media, Smartphone use and Self-Harm (3S-YP) study is a prospective observational cohort study to investigate the associations between social media and smartphone use and self-harm in young people. Young people aged 13–25 years old from secondary mental health services were recruited and followed for up to 6 months. Data collected in the study includes questionnaire data and data extracted from electronic health records (EHR) and user generated data sources.