BACKGROUND: Digital HIV interventions (DHI) have been efficacious in reducing sexual risk behaviors among sexual minority populations, yet challenges in promoting and sustaining users' engagement in DHI persist. Understanding the correlates of DHI engagement and their impact on HIV-related outcomes remains a priority. This study used data from a DHI (myDEx) designed to promote HIV prevention behaviors among single young men who have sex with men (YMSM; ages 18-24 years) seeking partners online. OBJECTIVE: The goal of this study is to conduct a secondary analysis of the myDex project data to examine whether YMSM's online behaviors (eg, online partner-seeking behaviors and motivations) are linked to participants' engagement (ie, the number of log-ins and the number of sessions viewed). METHODS: We recruited 180 YMSM who were randomized into either myDEx arm or attention-control arm using a stratified 2:1 block randomization. In the myDEx arm, we had 120 YMSM who had access to the 6-session intervention content over a 3-month period. We used Poisson regressions to assess the association between YMSM's baseline characteristics on their DHI engagement. We then examined the association between the participants' engagement and their self-reported changes in HIV-related outcomes at the 3-month follow-up. RESULTS: The mean number of log-ins was 5.44 (range 2-14), and the number of sessions viewed was 6.93 (range 0-22) across the 3-month trial period. In multivariable models, the number of log-ins was positively associated with high education attainment (estimated Poisson regression coefficient [β]=.22; P=.045). The number of sessions viewed was associated with several baseline characteristics, including the greater number of sessions viewed among non-Hispanic YMSM (β=.27; P=.002), higher education attainment (β=.22; P=.003), higher perceived usefulness of online dating for hookups (β=.13; P=.002) and perceived loneliness (β=.06; P=.004), as well as lower experienced online discrimination (β=-.01; P=.007) and limerence (β=-.02; P=.004). The number of sessions viewed was negatively associated with changes in internalized homophobia (β=-.06; P<.001) and with changes in perceived usefulness of online dating for hookups (β=-.20; P<.001). There were no significant associations between the number of log-ins and changes in the participants' behaviors at the 90-day follow-up. CONCLUSIONS: DHI engagement is linked to participants' sociodemographic and online behaviors. Given the importance of intervention engagement in the intervention's effectiveness, DHIs with personalized intervention components that consider the individuals' differences could increase the overall engagement and efficacy of DHIs. TRIAL REGISTRATION: ClinicalTrials.gov NCT02842060; https://clinicaltrials.gov/ct2/show/NCT02842060.