BackgroundNo web-based interventions addressing sexual problems are available for young adult survivors of childhood cancer. AimThis study aimed to test the efficacy of a web-based psychoeducational intervention, Fex-Can Sex, to alleviate sexual problems in young adults treated for cancer during childhood. MethodThis randomized controlled trial tested the effects of a 12-week, self-help, web-based intervention. Young adults (aged 19–40) reporting sexual dysfunction were drawn from a population-based national cohort of childhood cancer survivors and randomized to either an intervention group (IG, n = 142) or a wait-list control group (CG, n = 136). The primary outcome was ‘Satisfaction with sex life’ assessed by the PROMIS® SexFS v 2.0. Secondary outcomes included other SexFS domains, body image (BIS), emotional distress (HADS), health-related quality of life (EORTC QLQ-C30), and sex-related self-efficacy. Surveys were completed at baseline (T0), directly after the intervention (T1), and three months later (T2). The effects of the intervention were tested using t-test and linear mixed models, including intention-to-treat (ITT) and subgroups analysis. Adherence was based on log data extracted from the website system. The intervention included an open-ended question about perceived sexual problems. ResultsNo effect of the intervention was found in the primary outcome. Regarding secondary outcomes, the IG reported less vaginal dryness (Lubrication subscale) than the CG at T1 (p = 0.048) and T2 (p = 0.023). Furthermore, at T1, the IG reported less emotional distress than the CG (p = 0.047). Subgroup analyses showed that those with greater sexual problems at T0 improved over time (T1 and T2), regardless of group allocation. Overall, adherence to the intervention was low and participants' activity levels did not change the results. Additionally, some members of the IG reported increased understanding and acceptance of their sexual problems. ConclusionThe Fex-Can Sex intervention shows potential to improve sexual function, especially among those with greater dysfunction. To increase adherence and effect, we recommend the intervention to be further developed including more tailored content. Clinical trial registrationISRCTN Registry, trial number: 33081791 (registered on November 27, 2019).