Over 50% of young adults (defined as individuals aged 18-25 years) with substance use disorders (SUDs) have at least 1 co-occurring psychiatric disorder, and the presence of co-occurring disorders worsens SUD outcomes. Treatment of both co-occurring psychiatric disorders and SUDs in young adults is imperative for optimal treatment, yet many barriers exist to achieving this goal. We present a series of evidence-informed principles of care for young adults with co-occurring psychiatric disorders derived by a workgroup of experts convened by Boston Medical Centers Grayken Center for Addiction. The 3 principles are as follows: (1) young adults should receive integrated mental health and addiction care across treatment settings; (2) care should be responsive to the needs of young adults exposed to trauma and other adverse childhood experiences; and (3) treatment programs should regularly assess and respond to the evolving mental health needs, motivations, and treatment goals of young adults with co-occurring disorders. Our guidance for each principle is followed by a review of the evidence supporting that principle, as well as practice considerations for implementation. More research among young adults is critical to identify effective treatments and service systems for those with co-occurring disorders.