Homeless young adults are more likely than their never-homeless counterparts to abuse substances and engage in sexual risk behaviors. This study evaluated the feasibility and preliminary effectiveness of OnTrack, a smartphone application to self-monitor substance use and sexual risk behaviors, plus a brief motivational intervention (BMI), in reducing substance use and sexual risk among homeless young adults. A randomized controlled pilot trial (N = 60) compared OnTrack + BMI to treatment as usual (TAU) at an inner-city crisis shelter for homeless young adults (aged 18-21 years). Participants were assessed at baseline and 2 weeks, 4 weeks, and 6 weeks after baseline to evaluate substance use and sexual risk behaviors. Kruskal-Wallis tests determined differences between baseline and postintervention assessments. Logistic regression models examined treatment effect on change in outcomes between baseline and postintervention follow-up assessment, controlling for baseline levels. Participants in OnTrack + BMI significantly reduced past 2-week number of drinks (p = .023), times used marijuana (p = .046), times engaged in unprotected sex (p = .012), and times used drugs before sexual activity (p = .019). No reductions of substance use or sexual risk behaviors were found among participants in TAU (all ps > .05). After adjusting for baseline levels of substance use and sexual risk, participants in OnTrack + BMI had significantly lower odds than those in TAU for drinking alcohol (adjusted OR = 0.14; p = .01) and having unprotected sex (adjusted OR = 0.151; p = .032). OnTrack + BMI is feasible and effective in reducing past 2-week alcohol use and unprotected sex among homeless young adults. A larger randomized trial is warranted to replicate and extend present results. (PsycInfo Database Record (c) 2020 APA, all rights reserved).