Emerging adults (ages 18-29) are among the fastest growing HIV+ populations worldwide. Thailand has the highest adult HIV seroprevalence in Asia, with high rates of HIV infection among vulnerable emerging adults. Pre-exposure prophylaxis (PrEP) is an efficacious HIV prevention strategy recommended for all persons at substantial risk for HIV. PrEP is highly effective when taken as prescribed, but uptake and adherence have been low, particularly among vulnerable emerging adults. Key barriers include stigma related to external self-expression, limited employment, and high-risk behaviors. Vulnerable emerging adults experience various forms of stigma that may have a synergistic influence on engagement in HIV prevention, including PrEP use and HIV testing. To address these challenges, we propose to develop and pilot a mHealth intervention to overcome stigma and promote use of PrEP. We will adapt the 4-session, Healthy Choices (HC) intervention for mHealth (mHC) to increase access and delivery in our target population. We will also adapt selected modules of the Finding Respect and Ending Stigma around HIV (FRESH) intervention into a client focused, digitized package to address stigmas that harm vulnerable emerging adults. Intervention development will be guided by ADAPT-ITT and informed by two conceptual models. These intervention components will be integrated to enhance the impact of mHC through FRESH content and will be delivered via an innovative intervention platform. Our primary aim is to develop mHC and FRESH content and assess their preliminary efficacy. We will use a Multiphase Optimization Strategy (MOST) to identify the most effective intervention components to address stigma and PrEP use in this population. The proposed study will have 3 phases. Phase I includes focus groups with Thai vulnerable emerging adults and healthcare providers to explore stigma, and barriers and facilitators of the use of HIV prevention services through thematic analysis to inform intervention adaptation. Phase II consists of adapting and beta testing mobile Healthy Choices and stigma reduction intervention, leveraging a community advisory board. In Phase III, we will conduct a MOST design-based trial to assess feasibility, acceptability, and preliminary efficacy of intervention components. One hundred and eight vulnerable emerging adults will be randomized to one of four experimental conditions: (1) Standard PrEP Counseling (control), (2) mobile Healthy Choices + PrEP counseling, (3) stigma reduction + PrEP counseling, and (4) mobile Healthy Choices + stigma reduction + PrEP Counseling. Feasibility and acceptability of the intervention will be assessed through intervention engagement data and the System Usability Scale. Preliminary outcomes will be assessed by evaluating stigma reduction, PrEP use, and HIV testing. Participants will complete assessments at baseline and at 3- and 6-months post-intervention. Upon R34 completion, we will have a highly innovative mHea...