BACKGROUND: New cases of human immunodeficiency virus (HIV) among young men who have sex with men (YMSM), aged 18 to 24, underscore the importance of developmentally-informed HIV programs for YMSM. We developed an online intervention focused on risk reduction strategies across different sexual partner types. Intervention activities focus on assisting YMSM reflect on their partner-seeking behaviors, develop sexual decision-making rules to reduce their HIV risks, and consider the adoption of HIV prevention behaviors. OBJECTIVE: This pilot, randomized controlled trial (RCT) aims to examine the feasibility, acceptability, and preliminary efficacy of a tailored, Web-based HIV prevention intervention for single YMSM. METHODS: We designed a prospective RCT of online-recruited cis-gender men (N=180) who reported recent unprotected anal intercourse, self-report as HIV negative or are unaware of their HIV status, and meet sexual partners through online dating apps. Individuals in the control arm receive an attention-control condition that includes HIV/sexually transmitted infection (STI) information currently available on sex education websites. Individuals in the intervention arm receive a 6-session Web-based program tailored on their demographic information, partner-seeking behaviors and relationship desires, and prior sexual attitudes and behaviors. This tailored content will match HIV prevention messages and safer sex skills with YMSM's outcome expectancies when meeting new partners and thereby help them consider how to integrate safer sex practices into different partner types. Study assessments are taken at baseline, 30-, 60-, and 90-day follow-ups. Intervention acceptability and preliminary efficacy will be explored in sexual risk behaviors and HIV/STI testing. RESULTS: The RCT launched in November 2016 and is ongoing. To date, 180 eligible individuals have been enrolled, consented, and randomized. Of the 120 individuals in the intervention arm, 51.7% (62/120) identify as non-Hispanic white and half of the control arm identifies as non-Hispanic white. There were no differences observed by arm for race and/or ethnicity, age, or sexual orientation. CONCLUSIONS: Although there are in-person evidence-based interventions with proven efficacy for YMSM, few HIV/STI prevention interventions delivered online exist. Online interventions may ease access to comprehensive HIV/STI education among YMSM and allow personalized content to be delivered. The online intervention that we developed, myDEx, aims to alleviate the gaps within HIV prevention for YMSM by utilizing tailored, Web-based content with the goal of developing skills for same-sex dating and relationship building, while reducing their risks for HIV/STI. TRIAL REGISTRATION: ClinicalTrials.gov NCT02842060; https://clinicaltrials.gov/ct2/show/NCT02842060 (Archived by WebCite at http://www.webcitation.org/6rcJdxF9v).