Introduction The coronavirus disease 2019 (COVID-19) outbreak, first reported in Wuhan, China, in December 2019, quickly hit the world in just one month, causing a global public health emergency. We aimed to investigate whether the COVID-19 pandemic caused a delay in the hospital admissions of breast cancer patients and diagnosis of breast cancer, thus increasing the tumor size and the stage of the disease. Materials and methods Included in the study were patients who underwent breast cancer surgery between 01/03/2019 and 01/03/2020 (pre-COVID-19, first period) and between 01/03/2020 and 01/03/2021 (post-COVID-19, second period). Three hundred and seventy patients with enough details were included, and details were analyzed retrospectively. Tumor characteristics of pre-COVID-19 breast cancer patients were compared with the tumor characteristics of post-COVID-19 breast cancer patients. Demographics, preoperative diagnosis, tumor properties, surgical procedure (breast-conserving surgery, modified radical mastectomy, simple mastectomy, skin-sparing mastectomy), tumor size, total lymph node number, metastatic lymph node number, locally advanced disease, metastatic disease, and neoadjuvant therapy were evaluated. Results The mean tumor size increased significantly in the post-COVID-19 primary surgery group (p=0.005). There is no significant relationship between the pre-COVID-19 and post-COVID-19 period and pT in the neoadjuvant received group (p>0.05). The presence of pT2+pT3+pT4 was statistically significantly higher in the post-COVID-19 primary surgery group (p=0.001). The mean value of metastatic lymph nodes dissected between pre-COVID-19 and post-COVID-19 primary surgery groups increased significantly (p=0.010). Pericapsular extension was higher in the post-primary surgery group (p=0.002). Conclusion During the COVID-19 outbreak, breast cancer patients have difficulty accessing healthcare services and hesitate to apply to hospitals to fear contracting the COVID-19 disease. This situation has led to delays in diagnosing breast cancer patients, increased tumor size and pT grade, increased number of metastatic lymph nodes, pericapsular extension, and the resulting disease often appearing in advanced sizes and stages.