Objective: Schools should design holistic sex education programmes to improve sex knowledge and enhance positive sexual attitudes among adolescents. A school-based social marketing sex education programme named "Starting from love -- Go! Go! Go!" was designed. This study determined the effectiveness of the intervention to promote adolescent sexual health in junior high schools in Southern Taiwan. Method: A one-group pretest-posttest design was utilised. Cluster random sampling was used to select participants (seventh-grade students aged 13-14 years) from 24 junior high schools. Results: A total of 1,407 seventh-grade participants completed the survey. Among the 1,407 participants, 714 were boys and 693 were girls. The results revealed that the total average sexual knowledge score of the participants was 1.55 points higher after the interventions, indicating significantly increased sexual knowledge (p < 0.001). The sexual attitudes of participants after the interventions significantly improved compared with those beforehand (p < 0.001). Moreover, the sexual knowledge and attitudes of participants exhibited a positive correlation (r = 0.357, p< 0.001). Conclusion: The proposed school-based social marketing sex education programme holds the potential to improve sexual knowledge and enhance positive sexual attitudes of the junior high school students and the educational efficacy of sex education with respect to sex-related concerns.