Sexually transmitted infections (STIs), including human immunodeficiency virus (HIV), continue to disproportionately affect Black adolescents. Those experiencing the psychological sequelae of mental illnesses are at increased risk. Here, we outline the development of an HIV/STI prevention intervention for heterosexually-active Black adolescents with mental illnesses. This research was guided by a psychosocial expansion of the Theory of Planned Behavior, nested within a broader social determinants of health framework. A youth community advisory board provided study oversight. Heterosexually-active Black adolescents aged 14 to 17 were recruited from community-based outpatient mental health providers for focus groups, surveys and two "dress rehearsals" of the intervention protocol ( = 68). The qualitative and quantitative findings indicated that knowledge, attitudes and skills related to consistent condom use, reduced number of sexual partners, routine HIV/STI testing, abstinence and emotion regulation were important areas to target. These elicitation data provided insight on the context of HIV/STI risk for the study population, and were used to develop the intervention content. "Project GOLD: We are Kings and Queens" was designed to be delivered over two days (three hours per day), with eight, 45-minute modules. The activities address behaviors, as well as cognitive, emotional, psychological and social processes associated with HIV/STI risk. Alongside evidence-based HIV/STI prevention strategies (e.g., sexual partner communication skills), the intervention activities are rooted in principles of Cognitive Behavioral Therapy. Dress rehearsal participants were in favor of the intervention and provided feedback on activity length, gender and cultural relevance, and strategies to sustain attention; this information was used to finalize the curriculum. In partnership with the community, we developed a theoretically-driven, gender and culturally relevant, developmentally and psychologically appropriate HIV/STI prevention program. Our ultimate goal is to standardize sexual health assessment and intervention in outpatient mental health treatment settings to meet sexual health needs in this underserved population.