Young men who have sex with men are experiencing an ongoing HIV epidemic in Vietnam, with an HIV prevalence of 12%. However, HIV testing and pre-exposure prophylaxis (PrEP) use among young men who have sex with men remain low. To improve HIV testing and PrEP use, mobile health (mHealth) interventions hold great promise to reach young men who have sex with men in low- and middle-income countries. HealthMindr, an evidence-based mHealth intervention developed in the United States for men who have sex with men, is an ideal option for adaptation for young men who have sex with men in Vietnam. The specific aims are to (1) Adapt the HealthMindr app to improve HIV testing and PrEP uptake among young men who have sex with men in Vietnam; (2) Examine acceptability, feasibility and preliminary effectiveness of the adapted app among young men who have sex with men in Vietnam and (3) Explore barriers and facilitators of implementing the adapted app among young men who have sex with men in Vietnam. In Aim 1, theater testing will be conducted through focus group discussions with young men who have sex with men and key other stakeholders, including health officials, clinic staff and community-based organizations. Theater testing data will be used to refine the app for beta testing. During beta testing, a small group of young men who have sex with men will use the app for 30 days, then complete an online survey and an exit interview to rate the app's usability, provide their perspectives of the app and report any other areas for improvement. App analytics will also be collected to understand whether and how the app is used. In Aim 2, the adapted app with be tested among young men who have sex with men through a randomized controlled design. Young men who have sex with men will be randomized into the intervention or control arm (1:1 ratio). The intervention arm will have full access to the app for 3 months, while the control group will receive standard-of-care HIV services. Key app features include HIV test planning, risk and PrEP eligibility assessment, ordering of free condoms and HIV test kits, information about PrEP, and PrEP provider and HIV testing locations. HIV testing and PrEP use will be assessed at baseline and 3 months and compared between groups. A mixed methods approach will be used to evaluate app acceptability and feasibility. Quantitative data on acceptability and feasibility will be collected through the 3-month online survey, while qualitative data will be collected through in-depth interviews with a subset of young men who have sex with men in the intervention arm and other key stakeholders. In Aim 3, through the same qualitative study as Aim 2, factors that influence the implementation of the app will be explored. Determinants from seven domains will be identified: condition, technology, value proposition, adopter system, health organization, wider system and embedding/adaption over time. Only when mHealth interventions are both effective and adopted can they maximize their potential to reduce HIV incidence among target populations. This proposal will generate data for designing a full-scale hybrid type 1 effectiveness-implementation trial to test the adapted app and identify implementation strategies to address the barriers of app implementation in low and middle-income countries.