Young adult men who have sex with men (YMSM) are at increased risk for HIV, especially minority YMSM. Pre-exposure prophylaxis (PrEP) is a breakthrough daily pill to prevent HIV. Consistent adherence is key to PrEP effectiveness, which is why the CDC recommends adherence support be provided to all PrEP patients. Mobile health can overcome barriers to the delivery of adherence support, particularly for YMSM who may be most in need of it due, at least in part, to their young age. We created a culturally- and developmentally-sensitive PrEP adherence mobile app (called "Dot") that was tailored for culturally-diverse young adult MSM. After formative research and usability testing, we conducted a 6-week pre-post study to evaluate the impact of Dot on self-reported PrEP adherence, PrEP treatment self-efficacy, PrEP knowledge, and intention to practice safe sex among culturally-diverse YMSM, age 20-29. At 6 weeks, there were significant improvements in PrEP adherence, PrEP self-efficacy, and intention to practice safe sex. PrEP knowledge scores did not significantly change. The Dot app proved feasible and effective at improving PrEP adherence among culturally-diverse YMSM. Moreover, the app had a high-degree of user appeal, which is foundational to success of an mhealth intervention.