Objective To evaluate the efficacy of an online psychosocial group intervention for parents of children with a chronic illness, in terms of anxiety and depression, and disease-related coping skills. Methods Parents (N = 73) participated in a parallel multicenter randomized controlled trial comparing an intervention group to a waitlist control group. In the group intervention Op Koers Online (English: On Track Online) parents learned how to use adaptive coping strategies taught with cognitive behavioral therapy and acceptance and commitment therapy techniques. Assessments (online questionnaires) took place at baseline (T0), 6-months (T1), and 12-months (T2) follow-up. Mixed-model analyses were performed to test the difference in change in outcomes between intervention (N = 34) and waitlist control group (N = 33). Results When compared with the waitlist control group, the intervention had a significant positive effect (p < .05) on changes in anxiety, depression, and total score T1 versus T0 (β = -.47 to -.51) and T2 versus T0 (β = -.39 to -.46), the coping skills open communication, relaxation, social support, acceptance, predictive control (β = .42–.88) and helplessness (β = -.47) T1 versus T0 and relaxation and positive thinking T2 versus T0 (β = .42–.53). Conclusions Parental anxiety and depression decreased, and use of adaptive coping skills improved after the intervention. The online character, the focus on parents themselves instead of on their child and the possibility for parents of children with rare illnesses to participate, are innovative and unique aspects of Op Koers Online for parents. The next step is to implement the intervention in clinical practice.