Objective: Fear 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). Methods: Eligible 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. Results: Of 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. Conclusions: The 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.