Young men who have sex with men (YMSM) are highly vulnerable to HIV. While pre-exposure prophylaxis (PrEP) has demonstrated effectiveness, adherence has been low among YMSM and difficult to measure accurately. In collaboration with a healthcare company, we configured an automated directly-observed therapy (aDOT) platform for monitoring and supporting PrEP use. Based on interest expressed in focus groups among 54 YMSM, we combined aDOT with an electronic sexual diary to provide feedback on level of protection during sex and to motivate app use. In an 8-week optimization pilot with 20 YMSM in San Francisco and Atlanta, the app was found to be highly acceptable, with median System Usability Scale scores in the "excellent" range (80/100). App use was high, with median PrEP adherence of 91% based on aDOT-confirmed dosing. Most (84%) participants reported the app helped with taking PrEP. These promising findings support further evaluation of DOT Diary in future effectiveness studies.