Abstract Background The coronavirus disease 2019 (COVID-19) pandemic (officially declared on the March 11, 2020), and the resulting measures, are impacting daily life and medical management of breast cancer patients and survivors. We evaluated to what extent these changes have affected quality of life, physical, and psychosocial well-being of patients previously or currently being treated for breast cancer. Methods This study was conducted within a prospective, multicenter cohort of breast cancer patients and survivors (Utrecht cohort for Multiple BREast cancer intervention studies and Long-term evaLuAtion). Shortly after the implementation of COVID-19 measures, an extra survey was sent to 1595 participants, including the validated European Organization for Research and Treatment of Cancer (EORTC) core (C30) and breast cancer- specific (BR23) Quality of Life Questionnaire (EORTC QLQ-C30/BR23) and Hospital Anxiety and Depression Scale (HADS) questionnaire. Patient-reported outcomes (PROs) were compared with the most recent PROs collected within UMBRELLA pre–COVID-19. The impact of COVID-19 on PROs was assessed using mixed model analysis, adjusting for potential confounders. Results 1051 patients and survivors (65.9%) completed the survey; 31.1% (n = 327) reported a higher threshold to contact their general practitioner amid the COVID-19 pandemic. A statistically significant deterioration in emotional functioning was observed (mean = 82.6 [SD = 18.7] to 77.9 [SD = 17.3]; P < .001), and 505 (48.0%, 95% confidence interval [CI] = 45.0% to 51.1%) patients and survivors reported moderate to severe loneliness. Small improvements were observed in quality of life and physical, social, and role functioning. In the subgroup of 51 patients under active treatment, social functioning strongly deteriorated (77.3 [95% CI = 69.4 to 85.2] to 61.3 [95% CI = 52.6 to 70.1]; P = .002). Conclusion During the COVID-19 pandemic, breast cancer patients and survivors were less likely to contact physicians and experienced a deterioration in their emotional functioning. Patients undergoing active treatment reported a substantial drop in social functioning. One in 2 reported loneliness that was moderate or severe. Online interventions supporting mental health and social interaction are needed during times of social distancing and lockdowns.