Objectives: Transgender youth are significantly more likely to need mental healthcare than cisgender youth due to their high exposure to discrimination and victimization – including within mental healthcare. Accordingly, transgender youth have relatively low care satisfaction and high treatment drop-out, further exacerbating extant mental health inequities. To reduce these inequities, mental health providers need knowledge and skills to enhance transgender youth’s treatment engagement and benefits. However, a comprehensive set of practices addressing the needs of transgender youth patients and their providers does not exist. Thus, the current study developed the Gender Affirming Psychotherapy (GAP) intervention.Methods: GAP was developed using human-centered design, a methodological approach for creating interventions that prioritizes the needs of key stakeholders, which in this study included mental health providers, transgender youth, and their parents. A scoping review of the literature and stakeholder focus groups were conducted to identify GAP, which encompasses core principles and skills to enhance mental health services for transgender youth.Results: GAP encompasses 26 principles and 39 skills, organized within 10 domains. All principles and skills were designed to be relevant for various provider types (e.g., psychiatrists, social workers) and to be flexibly adapted to meet diverse patient needs.Conclusions: GAP offers a scalable and flexible approach to addressing the growing mental healthcare needs of transgender youth. This study also suggests that human-centered design is a feasible and efficient method for developing interventions to address health inequities.