Importance: The unmet needs regarding symptom management of psychological distress among patients with breast cancer must be addressed. However, little evidence exists on effective interventions, such as nurse navigation. Objective: To compare the long-term effects of the REBECCA (Rehabilitation After Breast Cancer) nurse navigation intervention vs usual care in patients with breast cancer who were psychologically vulnerable. Design, Setting, and Participants: This parallel randomized clinical trial recruited and evaluated for eligibility adult female patients with newly diagnosed breast cancer and symptoms of psychological distress (distress score of ≥7 points on Distress Thermometer) at Rigshospitalet in Copenhagen, Denmark, from August 2017 to October 2019. This study continued the work of a pilot study, extending the follow-up to 18 months. Patients who met the inclusion criteria were randomized to either standard care or the REBECCA intervention. Intention-to-treat analyses were performed from June 2021 to October 2022. Interventions: Patients who were randomized to the REBECCA intervention received nurse navigation and symptom screening as well as standard care. Standard care included regular treatment, nurse support at chemotherapy and radiotherapy appointments, and municipality-based rehabilitation. Main Outcomes and Measures: The primary outcome was distress, as measured using the Distress Thermometer. The secondary outcomes included symptoms of anxiety, symptoms of depression, breast cancer-specific health-related quality of life, fear of recurrence, sleep, cognitive function, patient activation, pain, health behavior, body mass index, and need for support. Long-term effects at 6, 12, and 18 months were examined using mixed-effect models, adjusting for randomization strata of age and treatment modality. Results: A total of 309 female patients were included in the analysis, with 153 patients randomized to the standard care group and 156 patients randomized to the REBECCA intervention group. Mean (SD) age was 56 (11) years with only small between-group differences. Patients receiving the REBECCA intervention compared with standard care had reduced (although not significant) symptoms of distress, especially at the 12-month follow-up (estimated effect = -0.51 [95% CI, -1.05 to 0.04]; effect size [ES] = -0.49). Significant effects were seen for symptoms of depression at 6 months (estimated effect = -1.39 [95% CI, -2.33 to -0.44]; ES = -0.27), and breast cancer-specific health-related quality of life at 12 months (estimated effect = 4.03 [95% CI, 1.28- 6.77]; ES = 0.31). Nonsignificant reductions were seen for symptoms of anxiety at 6 months (estimated effect = -1.00 [95% CI, -1.95 to -0.06]; ES = -0.21) and 12 months (estimated effect = -1.01 [95% CI, -1.97 to -0.04]; ES = -0.21), and a nonsignificant increase was seen for patient activation at 18 months (estimated effect = 3.52 [95% CI, -0.09 to 7.12]; ES = 0.25). Stronger intervention effects were observed for younger age, low patient activation, less education, and low social support. Conclusions and Relevance: Results of this study indicate that patients with breast cancer who were psychologically vulnerable (ie, having moderate to high psychological distress) did not experience significant reduction in distress with nurse navigation. Further research is needed to develop the intervention's framework and investigate its potential use in clinical practice. Trial Registration: ClinicalTrials.gov Identifier: NCT03254875.