Objective: The aim of this study was to evaluate the efficacy of an internet-based treatment for Genito-Pelvic Pain/Penetration Disorder (GPPPD) which adversely affects women's sexuality and is associated with reduced well-being and mental health comorbidities. Method: Two-hundred women with GPPPD (no penetrative intercourse ≥6 months) were randomly allocated to the intervention group (IG) or a waitlist control group (WCG). The intervention included eight modules and one booster session and was delivered through an eHealth platform. Participants were supported by an eCoach regarding treatment adherence. The primary outcome was intercourse penetration behavior. Online assessments were scheduled at baseline (T1), after Session 8/12 weeks (T2), and 6 months (T3) after randomization. Intention-to-treat analyses were reported. Results: Significantly more participants (31.00%, n = 31/100) in the IG were able to have sexual intercourse at T2 compared to those in the WCG, 13.00%, n = 13/100; χ2(1) = 9.44, p < .01. At T3, still more participants in the IG had sexual intercourse (29%) compared to those in the WCG (20%) but the groups no longer differed significantly, χ2(1) = 2.19, p = .19. Genital pain, painful and noncoital penetration behavior, and negative penetration-related cognitions significantly improved with medium to large effects at T2 (d = 0.66−1.25) and small to large effects at T3 (d = 0.23−1.32), whereas fear of sexuality, overall sexual functioning, trait anxiety, and well-being improved with small to medium effects (T2: d = 0.20−0.49, T3: d = 0.23−0.46) in the IG compared to the WCG. On average, participants completed 79% of the intervention. Conclusion: Internet-based treatment has been shown to be effective for GPPPD symptoms and could therefore be a promising treatment modality. What is the public health significance of this article? This study suggests that internet-based treatment can be an acceptable and effective approach to enable intercourse penetration behavior and reduce associated symptoms of Genito-Pelvic Pain/Penetration Disorder in comparison to a waitlist condition. Internet-based treatment might be recommended for women preferring self-help and who cannot and will not access face-to-face therapy due to structural and attitudinal barriers.