Abstract Young men who have sex with men (YMSM), especially those who are Black and Hispanic/Latino, are disproportionately impacted by new HIV infections. Pre-exposure prophylaxis (PrEP) is one of the most effective HIV prevention tools, however its benefits are directly correlated with medication adherence to sustain adequate tenofovir (TFV) levels in blood. Recent studies (ATN 110 & 113) show that young MSM prescribed PrEP struggle with adherence, especially when healthcare appointments are reduced from monthly to every 3 months, and most fail to maintain protective drug levels via PrEP over time. This has prompted calls for youth- specific interventions consisting of peer support groups and frequent healthcare interactions for YMSM on PrEP. The proposed study responds directly these recommendations by developing and pilot testing a mobile peer support intervention to improve PrEP adherence and retention in PrEP care for YMSM ages 18-29 years, called “PrEP it Together” (or PrEP iT!). PrEP iT! is grounded in the principles of the Technology Adoption Model and the Information, Motivation, and Behavioral Skills (IMB) model and includes virtual peer interaction and support, youth-centered PrEP and sexual health information, adherence reminders and self-monitoring, text message follow-up for youth whose adherence may be faltering, healthcare appointment reminders, healthcare provider interactions, and gamification components to optimize engagement. First, we will develop the PrEP iT! mobile intervention through focus groups with YMSM and refine iterations through input from a community advisory board (CAB). Four focus groups will be conducted to gain feedback on an existing mobile platform from which PrEP iT! will be adapted. Next, a CAB of YMSM will provide regular feedback during the iterative intervention design process. Usability testing with 6 members of the target population will be conducted, culminating in a final version of PrEP iT! Second, in preparation for a subsequent R01 trial, we will conduct a pilot randomized controlled trial to assess the feasibility, acceptability and preliminary impact of the PrEP iT! intervention for YMSM. Eighty YMSM recently initiating or restarting PrEP will be randomized to receive either PrEP iT! or usual care for 6 months, with biological and self-reported outcomes collected at baseline, and 3- and 6-month follow-up time points. Intervention use data will be collected to assess which intervention components impact study outcomes. Exit interviews of 10 YMSM will be conducted to further refine the intervention for a larger trial. PrEP iT! is innovative because it is the first intervention to use mobile phone technology to facilitate virtual peer support for YMSM on PrEP. The study advances PrEP science by addressing critical gaps (adherence & retention) in PrEP care among YMSM, and is sustainable because it will be built on an open source platform that may be integrated into clinical and community settings ...