Objective: This study aimed to compare the effectiveness of adolescents' use of smartphone applications and educational booklets in an educational programme about sexually transmitted infections (STIs). Design: Non-equivalent control-group time-series design. Setting: The study was conducted in two public high schools located in two urban areas of South Korea. Methods: The following measures were developed using Roger's protection-motivation theory as the study's conceptual framework: STI knowledge, STI vulnerability, STI prevention self-efficacy and STI prevention intentions. Data from 88 senior students were analysed: 47 students in the smartphone application group (experimental) and 41 in the educational booklet group (control). The procedures consisted of a face-to-face lecture plus self-study and a period of self-study only. Participants were tested at three time points: pre-test, post-test 1 (immediately after the programme's completion) and post-test 2 (5 weeks after the programme's completion). Results: Significant group differences were found at different times in STI knowledge, vulnerability, prevention self-efficacy and prevention intentions. Smartphone applications were more effective in sustaining the effects of the educational programme than the educational booklets. Conclusion: Researchers and educators should examine the role of relevant learning materials in the promotion of adolescents' development of self-study skills and their knowledge acquisition.