Background Prostate cancer treatment often results in significant psycho‐sexual challenges for men following treatment; however, many men report difficulty in accessing appropriate care. Methods A randomized controlled trial was undertaken to assess the efficacy of a 10‐week self‐guided online psychological intervention called My Road Ahead (MRA) for men with localized prostate cancer in improving sexual satisfaction. Participants were randomized to 1 of 3 conditions MRA alone or MRA plus online forum, or forum access alone. Pre, post, and follow‐up assessments of overall sexual satisfaction were conducted. Mixed models and structural equation modeling were used to analyze the data. Results One hundred forty‐two men (mean age 61 y; SD = 7) participated. The majority of participants had undergone radical prostatectomy (88%) and all men had received treatment for localized prostate cancer. Significant differences were obtained for the 3 groups (P = .026) and a significant improvement in total sexual satisfaction was observed only for participants who were allocated to MRA + forum with a large effect size (P = .004, partial η2 = 0.256). Structural equation modeling indicated that increases in sexual function, masculine self‐esteem, and sexual confidence contributed significantly to overall sexual satisfaction for the MRA + forum plus forum condition. Conclusions This study is the first, to our knowledge, that has evaluated a self‐guided online psychological intervention tailored to the specific needs of men with prostate cancer. The findings indicate the potential for MRA to deliver support that men may not otherwise receive and also highlight the importance of psychological intervention to facilitate improved sexual outcomes.