HIV transmission rates in the United States have increased among men who have sex with men. However, there are no published randomized trials examining interventions to reduce sexual risk for HIV acquisition in males younger than 18 years. To determine the efficacy of MyPEEPS Mobile, a mobile-delivered HIV prevention intervention, to reduce sexual risk behavior in same-sex attracted young males. This was a national randomized clinical trial of the efficacy MyPEEPS Mobile vs a delayed intervention among males aged 13 to 18 years to prevent or reduce sexual risk for HIV acquisition. Study activities were completed through 4 study sites in Birmingham, Alabama; New York, New York; Seattle, Washington; and Chicago, Illinois. Study staff at each site met with participants in person or via video conferencing. Data were collected from June 1, 2018, to April 7, 2020, and analyzed from July to October 2021. The MyPEEPS Mobile intervention contains 21 online psychoeducational and skill-building modules, which participants completed over a 3-month period. Participants randomized to the intervention group received access to MyPEEPS Mobile for the first 3 months, while those randomized to the delayed intervention group received access at their 9-month visit after data for the primary efficacy analysis had been collected. The self-reported primary outcome was change in the number of condomless anal sex acts between study conditions. Secondary outcomes were change in the number of sex partners, number of condomless anal sex partners, the number of sex acts while under the influence of substances, preexposure prophylactic uptake, nonoccupational postexposure prophylaxis use, and HIV and sexually transmitted infection testing. In the analytic sample of 763 racially and ethnically diverse study participants, the mean (SD) age was 16.2 (1.4) years; 736 participants (97%) were male, 13 (2%) nonbinary; and 6 (1%) genderqueer; 158 (21%) were Black or African American, 311 (41%) were Hispanic or Latino, and 284 (37%) were White. Overall, 382 were randomized to the intervention group and 381 to the delayed intervention group. At 3-month follow-up, there was a significant reduction in the number of condomless anal sex acts in the intervention group compared with the delayed intervention group (incidence rate ratio [IRR], 0.56; 95% CI, 0.32-0.99); however, there was no significant difference between groups at 6 or 9 months. In subgroup analyses, the intervention effect was pronounced among Black non-Hispanic participants at 3-month follow-up (IRR, 0.19; 95% CI, 0.04-0.94) and 6-month follow-up (IRR, 0.15; 95% CI, 0.03-0.78) compared with the delayed intervention group. There were no significant differences in the change in the number of sex partners, number of condomless anal sex partners, the number of sex acts while under the influence of substances, preexposure prophylactic uptake, nonoccupational postexposure prophylaxis use, and HIV and sexually transmitted infection testing between the intervention and delayed intervention groups. In this study, the MyPEEPS Mobile intervention demonstrated a 44% overall reduction in condomless anal sex at 3-month follow-up compared with the delayed intervention group, but not at 6 or 9 months. To our knowledge, MyPEEPS Mobile is the first intervention to demonstrate evidence of short-term efficacy for reducing sexual risk among same-sex attracted young males. ClinicalTrials.gov Identifier: NCT03167606.