BACKGROUND: Sexual difficulties and vaginal pain are common following treatment for breast cancer. AIM: The goal of this study was to evaluate an online mindfulness-based group sex therapy vs an online supportive sex education group therapy to address these sexual difficulties. METHODS: Breast cancer survivors (n = 118) were randomized to 1 of the 2 arms; 116 provided informed consent and completed the time 1 assessment. Treatment included 8 weekly 2-hour online group sessions. Those randomized to the mindfulness group completed daily mindfulness exercises, and those in the comparison arm read and completed exercises pertaining to sex education. OUTCOMES: Assessments were repeated at posttreatment and 6 months after the completion of the group. RESULTS: There was a main effect of treatment on primary endpoints of sexual desire, sexual distress, and vaginal pain, with all outcomes showing significant improvements, with no differential impact by treatment arm. Secondary endpoints of interoceptive awareness, mindfulness, and rumination about sex also significantly improved with both treatments, with no group-by-time interaction. CONCLUSION: Both mindfulness-based sex therapy and supportive sex education delivered in group format online are effective for improving many facets of sexual function, vaginal pain, rumination, mindfulness, and interoceptive awareness in breast cancer survivors. STRENGTHS AND LIMITATIONS: We used a randomized methodology. Future studies should seek to diversify participants. CLINICAL IMPLICATIONS: These findings highlight the need to offer similar treatments to more breast cancer survivors immediately after and in the years following cancer treatment as a means of improving survivorship quality of life.