AbstractObjectiveFear of cancer recurrence (FCR) is a distressing concern among cancer survivors. Interventions to address FCR need to be effective but also accessible and low cost. This randomized controlled trial evaluated the efficacy of an online group‐based psychological intervention for FCR (ConquerFear‐Group).MethodsEligible breast cancer (BC) survivors had completed primary treatment 3 months–5 years previously, were ≥18 years, and scored ≥22 on the Fear of Cancer Recurrence Inventory–Short Form (FCRI‐SF). Participants were randomized to online ConquerFear‐Group (focusing on metacognitive strategies, values‐clarification, and education about follow‐up behavior) or online group‐based relaxation training (active control). Questionnaires were completed at baseline (T1), 1 week post‐intervention (T2), three (T3) and six (T4) months later. The primary outcome was FCR (FCRI total). A number of secondary and process outcomes were also collected. Treatment effects were evaluated with mixed linear models.ResultsOf 866 eligible BC survivors, 475 (55%) completed the FCR screening, and 85 (18%) were randomized to ConquerFear‐Group or relaxation training (2 × 6 groups). Compared with control participants, ConquerFear‐Group participants experienced larger reductions in FCR (Cohen's d = 0.47, p = 0.001) and FCR severity (d = 0.57, p < 0.001), as well as mindfulness and decentering from baseline through follow‐up, and improvements in emotion regulation (T2), worry (T2, T3) and rumination (T2) at some time points.ConclusionsThe results demonstrated statistically significant and stable effects of ConquerFear‐Group on FCR that were maintained over a 6‐month period. It is suggested to investigate the program in a real‐life setting, where a pragmatic trial can further demonstrate feasibility and effectiveness.