PROJECT ABSTRACT Transgender youth are 2-3 times more likely to be diagnosed with depression or anxiety and 6 times more likely to attempt suicide relative to cisgender youth. These mental health inequities are exacerbated by mental healthcare barriers, such as discrimination. Negative mental healthcare experiences are associated with lower treatment engagement for transgender youth, including high dropout and low satisfaction. Conversely, mental healthcare that is gender-affirming - that which addresses the needs and experiences of transgender people - is associated with strong treatment engagement and better outcomes, relative to non-affirming treatments. Despite the positive impacts of gender-affirming mental healthcare, it is rarely provided. To address this void in mental healthcare, we developed and pilot-tested Gender-Affirming Psychotherapy (GAP), an intervention consisting of research-informed principles and skills that augment mental health treatment to address transgender youth’s needs. Examples of intervention skills include: using affirmed names, using preferred pronouns, and avoiding invasive questions. In a pilot trial situated in a medium-sized mental health clinic (K23MH124670), GAP demonstrated strong acceptability, feasibility, and appropriateness. A 10-hour implementation package (i.e., combination of strategies) encompassed within a self-paced online training, was also delivered across 2 months. The package included 5 discrete implementation strategies: (1) didactics, (2) patient stories, (3) practice, (4) incentives, and (5) an organizational support message. It demonstrated acceptability and feasibility, and was effective in improving provider attitudes, knowledge, self-efficacy (implementation mechanisms) and GAP adoption (implementation outcome). As the next step in efficiently and pragmatically promoting mental health equity for transgender youth, we will conduct a trial to optimize an implementation package to increase GAP adoption in large healthcare systems in places with varying transphobia levels. In Aim 1, we will validate the conceptual model of GAP implementation and effectiveness for large healthcare systems, using data from n=726 providers and n=1071 transgender patients from two large healthcare systems. In Aim 2, we will identify an optimized GAP implementation package, which will be the package with the fewest costs relative to implementation success. Finally, in Aim 3, we will explore structural transphobia as a moderator of GAP implementation and effectiveness to determine whether GAP implementation and/or intervention needs differ in high (vs. low) structural transphobia regions.