Drowning among young adults is high in Sri Lanka. Water safety education is a recommended strategy for drowning prevention but is often overlooked for young adults. This study aimed to evaluate the effectiveness of an adapted educational intervention, “Swim for Safety” on improving water safety knowledge, attitudes and survival swimming skills among undergraduates (19–28 years) in Sri Lanka. This study employed a parallel-group, two-arm randomized controlled trial design. The intervention group (n = 78) received a face-to-face, 12-lesson education programme, and the control group (n = 78) received a brochure and weekly mobile phone messages for six consecutive weeks. Baseline, post-intervention and three-month follow-up knowledge, attitudes and skills were evaluated. Knowledge and attitudes were assessed using a self-administered questionnaire and skills were evaluated following a skills assessment protocol. In total 116 participants, 60 intervention group and 56 control group, completed the study. At baseline there were no differences between groups in median scores of water safety knowledge, attitudes and survival swimming skills. The intervention group demonstrated statistically significant increases in median water safety knowledge, attitudes and survival swimming skill scores compared with the control group, following the intervention and maintained at three-month follow-up (p < 0.05). The adapted Swim for Safety programme significantly improved water safety knowledge, attitudes, and survival swimming skills among young adults in Sri Lanka. Therefore, it is recommended that the SfS programme be implemented widely to prevent drowning in young adults.