Background Efficacious mental health interventions for sexual and gender minority youth have had limited reach, given their delivery as time-intensive, in-person sessions. Internet-based interventions may facilitate reach to sexual and gender minority youth; however, there is little research examining their efficacy. Objective This study aims to describe the results of a pilot randomized controlled trial of imi, a web application designed to improve mental health by supporting lesbian, gay, bisexual, transgender, queer, and other sexual and gender minority identity affirmation, coping self-efficacy, and coping skill practice. Methods Sexual and gender minority youth (N=270) aged 13 to 19 (mean 16.5, SD 1.5) years and living in the United States were recruited through Instagram advertisements. Approximately 78% (210/270) of the sample identified as racial or ethnic minorities. Participants were randomized in a 1:1 fashion to the full imi intervention web application (treatment; 135/270, 50%) or a resource page–only version of the imi site (control; 135/270, 50%). The imi application covered four topical areas: gender identity; lesbian, gay, bisexual, transgender, queer, and other sexual and gender minority identity; stress and coping; and internalized homophobia and transphobia. Participants explored these areas by engaging with informational resources, exercises, and peer stories at a self-guided pace. Both arms were assessed via web-based surveys at baseline and 4-week follow-up for intervention satisfaction, stress appraisals (ie, challenge, threat, and resource), coping skills (ie, instrumental support, positive reframing, and planning), and mental health symptoms among other outcomes. Main intent-to-treat analyses compared the arms at week 4, controlling for baseline values on each outcome. Results Survey retention was 90.4% (244/270) at week 4. Participants in the treatment arm reported greater satisfaction with the intervention than participants in the control arm (t241=–2.98; P=.003). The treatment arm showed significantly greater improvement in challenge appraisals (ie, belief in one’s coping abilities) than the control (Cohen d=0.26; P=.008). There were no differences between the arms for threat (d=0.10; P=.37) or resource (d=0.15; P=.14) appraisals. The treatment arm showed greater increases in coping skills than the control arm (instrumental support: d=0.24, P=.005; positive reframing: d=0.27, P=.02; planning: d=0.26, P=.02). Mental health symptoms improved across both the treatment and control arms; however, there were no differences between arms. Within the treatment arm, higher engagement with imi (≥5 sessions, >10 minutes, or >10 pages) predicted greater improvement in stress appraisals (all P values <.05). Conclusions The results provide initial evidence that asynchronous psychosocial interventions delivered via a web application to sexual and gender minority youth can support their ability to cope with minority stress. Further research is needed to examine the long-term effects of the imi application. Trial Registration ClinicalTrials.gov NCT05061966; https://clinicaltrials.gov/ct2/show/NCT05061966