Purpose We evaluated the effect of Internet-based cognitive behavioral therapy (CBT) on sexual functioning and relationship intimacy (primary outcomes) and body image, menopausal symptoms, marital functioning, psychological distress, and health-related quality of life (secondary outcomes) in breast cancer survivors (BCSs) with a DSM-IV diagnosis of a sexual dysfunction. Patients and Methods We randomly assigned 169 BCSs to either Internet-based CBT or a waiting-list control group. The CBT consisted of weekly therapist-guided sessions, with a maximum duration of 24 weeks. Self-report questionnaires were completed by the intervention group at baseline (T0), midtherapy (T1), and post-therapy (T2) and at equivalent times by the control group. We used a mixed-effect modeling approach to compare the groups over time. Results Compared with the control group, the intervention group showed a significant improvement over time in overall sexual functioning (effect size for T2 [ES_T2] = .43; P = .031), which was reflected in an increase in sexual desire (ES_T1 = .48 and ES_T2 = .72; P < .001), sexual arousal (ES_T2 = .50; P = .008), and vaginal lubrication (ES_T2 = .46; P = .013). The intervention group reported more improvement over time in sexual pleasure (ES_T1 = .32 and ES_T2 = .62; P = .001), less discomfort during sex (ES_T1 = .49 and ES_T2 = .66; P = .001), and less sexual distress (ES_T2 = .59; P = .002) compared with the control group. The intervention group reported greater improvement in body image (ES_T2 = .45; P = .009) and fewer menopausal symptoms (ES_T1 = .39; P = .007) than the control group. No significant effects were observed for orgasmic function, sexual satisfaction, intercourse frequency, relationship intimacy, marital functioning, psychological distress, or health-related quality of life. Conclusion Internet-based CBT has salutary effects on sexual functioning, body image, and menopausal symptoms in BCSs with a sexual dysfunction.