Background Chemsex—the use of psychoactive drugs to enhance the sexual experience—is an increasing phenomenon globally. Despite the increasing burden and associated harms of chemsex, evidence-based interventions (ie, behavioral and pharmacological) for chemsex users are nonexistent. Objective In this study, we assessed the usability and acceptability of a mobile health (mHealth)–delivered safer chemsex package (“PartyPack”) as a sexual harm reduction strategy among men who have sex with men in Malaysia—a setting where chemsex is becoming increasingly prevalent. Methods This study is part of a larger smartphone app-based intervention (ie, JomPrEP; University of Connecticut) designed to improve access to HIV prevention services among Malaysian men who have sex with men. A total of 50 participants were recruited from the Greater Kuala Lumpur region of Malaysia to use the JomPrEP app, which included a feature allowing participants to order PartyPack, for 30 days (March-April 2022). The usability and acceptability of the PartyPack were assessed using self-report, app analytics, and exit interviews (n=20). Results Overall, 8% (4/50) of participants reported having engaged in chemsex in the past 6 months; however, engagement in condomless sex (34/50, 68%) and group sex (9/50, 18%) was much higher. A total of 43 (86%) participants ordered PartyPack, of which 27 (63%) made multiple orders during the 30 days. Most participants (41/43, 95%) reported being satisfied with the PartyPack order feature in the app, with 91% (39/43) indicating the order and tracking process was easy. Thematic data exploration further revealed important information for understanding (eg, items included in the package, use of mHealth platform to order package, and discreetness of the PartyPack box and order and delivery) and refining the logistical preferences (eg, using branded items and allowing customization during order). Conclusions Our findings provide strong evidence of the usability and acceptability of a mHealth-delivered safer chemsex package as a potential sexual harm reduction tool among this underserved population. Replication in a study with a larger sample size to test the efficacy of the PartyPack is warranted.