Adolescence is a critical life stage with significant effects on lifestyle in later life. Promoting adolescents’ lifestyle requires the use of appropriate educational approaches. This study aimed to evaluate the effects of face-to-face education for student health ambassadors (SHA) on the health-promoting lifestyle of adolescent female students. This randomized controlled trial was conducted in 2018. A random sample of 208 adolescent female students was recruited from public schools for girls in Tehran, Iran, and allocated through cluster randomization to a control and an intervention group. Twelve students were selected from the intervention group as SHAs and were provided with face-to-face training about health-promoting lifestyle in five one-hour sessions. Then, they provided indirect education about health-promoting lifestyle to their peers during daily school activities and conversations. Prior to and two months after the study intervention, participants’ health-promoting lifestyle was assessed using the Adolescent Health Promotion Short Form. The SPSS program (v. 24.0) was used for data description, and the Stata program (v. 13.0) was used for data analysis using the independent-samples t test, the chi-square test, and linear regression analysis. There were no significant between-group differences with respect to the pretest mean scores of health-promoting lifestyle and its dimensions (P > 0.05). However, the post-test mean scores a of health-promoting lifestyle and its dimensions in the intervention group were significantly greater than the control group (P < 0.001). Face-to-face education for SHAs significantly improves the success of their positive influence on the health-promoting lifestyle of adolescent female students. Integration of face-to-face training in ambassador-based health-promotion programs is recommended to increase the effectiveness of such programs.