Background: The focus of this review was to assess changes in youth mental health, psychological wellbeing, or substance use, as well as changes or disruptions to the delivery of mental health or substance use services for young people during the COVID-19 pandemic. Method: We conducted a rapid review of the literature on our outcomes of interest among youth (age <=25) in the context of the COVID-19 pandemic. Primary studies and systematic reviews on change were eligible for inclusion. Searches were conducted in PubMed and Embase in May 2021, and two reviewers screened studies for inclusion. We report results using a narrative synthesis.Results: We included 156 primary publications. A variety of methods were used to assess change, including prospective assessment of longitudinal cohorts, retrospective recall by participants in cross-sectional and qualitative studies, and comparison of peri-pandemic data to pre-pandemic normative values. Publications regarding mental health (n = 121) and psychological wellbeing (n = 26) generally indicated poor outcomes during the pandemic period. Publications on substance use (n = 41) revealed overall declines or unchanged patterns of use, though certain groups reported increased or problematic use. Studies of service delivery (n = 11) indicated a generally positive reception for helplines and telehealth, although some youth experienced difficulties accessing mental health services. Conclusions: The evidence indicates negative impacts of the COVID-19 pandemic on young people’s mental health, although declines in alcohol and nicotine use were also found. Services will need to continue to adapt as the pandemic progresses, particularly to support disadvantaged youth who lack access to telehealth resources.