The purposes of the study was to determine the effectiveness of peer-led and parent-led education on menstrual hygiene-related knowledge of in-school adolescent girls in Ogun state, Nigeria, which was conceptualized using the Health Belief Model. The study employed a quasi-experimental design with 120 in-school adolescent girls selected through a multistage sampling technique from four (4) secondary schools within Ogun East. A validated instrument was used for data collection. The findings showed that the adolescent girls' level of menstrual hygiene knowledge had a significant increase after the intervention. The peer-led group better predicted higher knowledge scores (mean diff = 7.13; effect size = 4.34; t = 19.091; p = 0.000). In conclusion, the peer-led education intervention was more effective in improving the level of adolescent girls' menstrual hygiene knowledge in Ogun State. It is recommended that peer educators be recruited for future interventions to change the reproductive health behaviors of adolescent girls nationally.