BACKGROUND: Men who have sex with men (MSM) in China are disproportionately affected by the HIV epidemic, and medication adherence to antiretroviral treatment in this vulnerable population is suboptimal. To address this issue, we developed an app-based case management service with multiple components, informed by the Information Motivation Behavioral skills model. OBJECTIVE: We aimed to conduct a process evaluation for the implementation of an innovative app-based intervention guided by the Linnan and Steckler framework. METHODS: Process evaluation was performed alongside a randomized controlled trial in the largest HIV clinic in Guangzhou, China. Eligible participants were HIV-positive MSM aged ≥18 years planning to initiate treatment on the day of recruitment. The app-based intervention had 4 components: web-based communication with case managers, educational articles, supportive service information (eg, information on mental health care and rehabilitation service), and hospital visit reminders. Process evaluation indicators of the intervention include dose delivered, dose received, fidelity, and satisfaction. The behavioral outcome was adherence to antiretroviral treatment at month 1, and Information Motivation Behavioral skills model scores were the intermediate outcome. Logistic and linear regression was used to investigate the association between intervention uptake and outcomes, controlling for potential confounders. RESULTS: A total of 344 MSM were recruited from March 19, 2019, to January 13, 2020, and 172 were randomized to the intervention group. At month 1 follow-up, there was no significant difference in the proportion of adherent participants between the intervention and control groups (66/144, 45.8% vs 57/134, 42.5%; P=.28). In the intervention group, 120 participants engaged in web-based communication with case managers and 158 accessed at least 1 of the delivered articles. The primary concern captured in the web-based conversation was the side effects of the medication (114/374, 30.5%), which was also one of the most popular educational articles topics. The majority (124/144, 86.1%) of participants that completed the month 1 survey rated the intervention as "very helpful" or "helpful." The number of educational articles accessed was associated with adequate adherence in the intervention group (odds ratio 1.08, 95% CI 1.02-1.15; P=.009). The intervention also improved the motivation score after adjusting for baseline values (β=2.34, 95% CI 0.77-3.91; P=.004). However, the number of web-based conversations, regardless of conversation features, was associated with lower motivation scores in the intervention group. CONCLUSIONS: The intervention was well-received. Delivering educational resources of interest may enhance medication adherence. The uptake of the web-based communication component could serve as an indicator of real-life difficulties and could be used by case managers to identify potential inadequate adherence. TRIAL REGISTRATION: Clinicaltrial.gov NCT03860116; https://clinicaltrials.gov/ct2/show/NCT03860116. INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID): RR2-10.1186/s12889-020-8171-5.