<sec> <title>BACKGROUND</title> <p>Many youths with mental health needs are unable to access care. Single-session interventions (SSIs) have helped reduce youth psychopathology across multiple trials, promising to broaden access to effective, low-intensity supports. Online, self-guided SSIs may be uniquely scalable, particularly if they are freely available for as-needed use. However, the acceptability of online SSI and their efficacy have remained unexamined outside of controlled trials, and their practical utility is poorly understood.</p> </sec> <sec> <title>OBJECTIVE</title> <p>We evaluated the perceived acceptability and proximal effects of Project YES (Youth Empowerment &amp; Support), an open-access platform offering three online SSIs for youth internalizing distress.</p> </sec> <sec> <title>METHODS</title> <p>After selecting one of three SSIs to complete, participants (ages 11-17 years) reported pre- and post-SSI levels of clinically relevant outcomes that SSIs may target (eg, hopelessness, self-hate) and perceived SSI acceptability. User-pattern variables, demographics, and depressive symptoms were collected to characterize youths engaging with YES.</p> </sec> <sec> <title>RESULTS</title> <p>From September 2019 through March 2020, 694 youths accessed YES, 539 began, and 187 completed a 30-minute, self-guided SSI. SSI completers reported clinically elevated depressive symptoms, on average, and were diverse on several dimensions (53.75% non-white; 78.10% female; 43.23% sexual minorities). Regardless of SSI selection, completers reported pre- to post-program reductions in hopelessness (d_av=0.53; d_z=0.71), self-hate (d_av=0.32; d_z=0.61), perceived control (d_av=0.60; d_z=0.72) and agency (d_av=0.39; d_z=0.50). Youths rated all SSIs as acceptable (eg, enjoyable, likely to help peers).</p> </sec> <sec> <title>CONCLUSIONS</title> <p>Results support the perceived acceptability and utility of open-access, free-of-charge SSIs for youth experiencing internalizing distress.</p> </sec> <sec> <title>CLINICALTRIAL</title> <p>Open Science Framework; osf.io/e52p3</p> </sec>