PURPOSE: Using a community-engaged approach, we adapted a human immunodeficiency virus (HIV) prevention smartphone app, Transpire, to meet the HIV and sexually transmitted infection (STI) prevention needs of transgender men and other transmasculine people. We conducted a pilot study to assess the feasibility and acceptability of the app among participants in two cities in the southeastern United States. METHODS: Participants were recruited online and through community partners. Eligible and consenting participants used the Transpire app for 3 months and completed surveys at the beginning and end of the follow-up period. Transpire gave participants access to HIV and STI prevention information; the ability to order HIV and STI at-home self-test kits, condoms, and lube; and mental health and substance use resources. The primary feasibility outcome was the System Usability Scale (SUS), which measures the usability of technology-based products. RESULTS: Sixty participants were enrolled. Most were 18-24 years old (=33, 55%) and identified as non-Hispanic white (=46, 77%). The mean SUS score was 72.4, which meets established criteria for acceptability. Fifteen (32%) participants conducted an HIV test during the 3-month follow-up period and 2 (4%) initiated pre-exposure prophylaxis. CONCLUSIONS: Following a community-engaged adaptation process, the Transpire app was found to be acceptable in a pilot study conducted among transmasculine participants. These pilot data indicate that app-based interventions have the potential to increase awareness of and access to sexual health resources for transmasculine people. Additional studies will be needed to assess the efficacy of Transpire or similar apps to improve sexual health outcomes.