BACKGROUND: Adolescent girls engaged in sexual activity are unlikely to use condoms during their sexual debut. Younger adolescents are also not often targeted for sexual-related interventions because of the taboos associated with sexuality in traditional societies. This study examined the association between peer-group participation and condom use at sexual debut among young adolescent girls in rural Eastern Ethiopia. DESIGN: The study used and analyzed data from end line survey of an implementation study involving 3,290 young adolescent girls aged 13-17 years. The intervention specifically targeted adolescent girls between the ages of 10 and 14 years who were part of a peer group. Unmarried sexually active adolescent girls and married adolescent girls who had their sexual debut before marriage were the study population. Multi-level mixed-effect logistic regression analysis was employed to examine associations using STATA/SE version 16 statistical software. RESULTS: Among 3,290 adolescent girls surveyed, 258 (7.84%) reported engaging in sexual intercourse. The mean age (SD) age at sexual debut was 14.36 (+1.32), with no observed statistical difference between the intervention and control groups (p=0.1164). The magnitude of condom use at sexual debut was 22.46%, 95 % CI (14.11, 33.81%). Adolescent girls who participated in peer-groups had 11.51 (Adjusted OR: 11.51, 95% CI: 1.95, 67.84) higher odds of using condoms during sexual debut compared to those in the control group. CONCLUSIONS: Peer group participation improved condom use at sexual debut. Peer groups can be critical for engaging adolescent girls in HIV and other sexually transmitted infection prevention and avoiding unwanted/ unplanned pregnancies. Further studies with larger sample size and specific design methodologies are imperative to gather robust evidence that support scale-up of this potentially lifesaving intervention.