The United States spends more money on mental health services than any other country, yet access to effective psychological services remains strikingly low. The need-to-access gap is especially wide among children and adolescents, with up to 80% of youths with mental health needs going without services, and the remainder often receiving insufficient or untested care. Single-session interventions (SSIs) may offer a promising path toward improving accessibility, cost-effectiveness, and completion rates for youth mental health services. SSIs are structured programs that intentionally involve only one visit or encounter with a clinic, provider, or program; they may serve as stand-alone or adjunctive clinical services. A growing body of evidence supports the capacity of SSIs to reduce and prevent youth psychopathology of multiple types. Here, we provide a working definition of SSIs for use in future research and practice; summarize the literature to date on SSIs for child and adolescent mental health; and propose recommendations for the future design, evaluation, and implementation of SSIs across a variety of settings and contexts. We hope that this paper will serve as an actionable research agenda for gauging the full potential of SSIs as a force for youth mental health.