Introduction Difficulties with vaginal penetration can severely affect a woman's desire to have sexual intercourse, her sexual and general well-being, or her partnership. However, treatment opportunities for vaginismus are scarce. Aim To evaluate the efficacy of an internet-based guided self-help intervention for vaginismus in a randomized controlled pilot trial. Methods Seventy-seven women with vaginismus (primary inclusion criterion = no intercourse ≥ 6 months) were randomly assigned to an intervention group (IG) and a waitlist control group (WCG). The intervention consisted of 10 sessions involving psychoeducation, relaxation exercises, sensate focus, and gradual exposure with dilators. Participants received written feedback on completed sessions from an eCoach. Main Outcome Measures The primary outcome was successful sexual intercourse. Secondary outcomes were non-intercourse penetration, fear of coitus, sexual functioning, and dyadic coping. Self-reported assessments were scheduled at baseline, 10 weeks, and 6 months. Results More participants (10 of 40, 34.48%) in the IG had intercourse compared with those in the WCG (6 of 37, 20.69%) at least once at 10 weeks or 6 months (odds ratio = 2.02). The difference was not significant (χ²₁ = 1.38, P = .38), but in the IG, there was a significant increase in intercourse penetration from baseline to 6 months (d = 0.65). No such increase was found in the WCG (d = 0.21). There were significant between-group effects concerning non-intercourse penetration (self-insertion of a finger or dilator or insertion by the partner) in favor of the IG. Fear of coitus and dyadic coping significantly decreased in the IG. Overall satisfaction with the training was high. Conclusion This randomized controlled trial showed promising effects of an internet-based intervention by increasing participants' ability to have intercourse and non-intercourse penetration while experiencing high treatment satisfaction. The WCG also showed improvement, although participants had vaginismus for an average duration of 6 years. Internet-based interventions could be a treatment modality to complement other methods in stepped care for vaginal penetration difficulties.