Objective: The coronavirus disease-2019 (COVID-19) pandemic causes delays in the diagnosis and treatment of cancer patients due to fear of contagion and lockdown. This study aims to investigate the effects of the COVID-19 pandemic on breast cancer patients treated in our breast center. Materials and Methods: Patients who applied to our clinic with the diagnosis of invasive breast cancer in March 2020 and March 2021 (Study Group) when the COVID-19 pandemic was observed, and in March 2019 and March 2020 before the COVID-19 pandemic (Control Group) were compared in terms of demographic, clinical and pathological characteristics. Statistical analyses were performed using the SPSS software version 21. Results: There were 176 (46%) patients in the study and 206 (54%) patients in the control group. Almost a 15% reduction was detected in patients admitted during the COVID-19 pandemic. The rate of pre-menopausal patients and patient-related delay time (PRDT) were significantly higher in SG (57.7% vs. %45, p=0.013, 2.58 vs. 1.82-month, p=0.001, respectively). There was a larger tumor size and more metastatic lymph nodes after NAC in the SG, but the differences were not significant. There was no difference regarding breast cancer stages and molecular subtypes between the two groups, but there was significantly more de novo stage IV breast cancer in the SG (p=0.009). The incidence of neo-adjuvant chemotherapy and type of surgical therapy was similar between the two groups. Conclusion: COVID-19 pandemic caused a decrease in the number of patients who applied to our clinic and increased patient-related delay time due to fear of transmission and lockdown. The rate of de novo stage IV breast cancer was also significantly increased.