The Cultural Adaptation of a Substance Use Treatment (CAST) Project is a community-led mixed-methods study focused on developing a culturally responsive substance use intervention for Black youth impacted by the legal system. The study examines how racial discrimination influences substance use and mental health outcomes while centering the perspectives of youth, parents, and community stakeholders. Participants include Black youth (n=15), parents (n=10), and community members (n=10), who will participate in focus groups to inform treatment adaptations. Using the ADAPT-ITT framework, the project will culturally adapt Motivational Enhancement Therapy and Cognitive Behavioral Therapy (MET/CBT12). An open feasibility trial will evaluate the adapted intervention. Primary outcomes include feasibility and acceptability, while secondary outcomes include changes in substance use, discrimination-related distress, and mental health symptoms.