Introduction: The outbreak of COVID-19 has had an impact on global healthcare as well as on radiotherapy practice in many countries. This study aimed to identify clinical characteristics of Coronavirus Disease 2019 (COVID-19) infected cancer inpatients, as well as what impact this infection had on radiation treatment of the patients. Methodology: In this retrospective study, we included cancer inpatients with laboratory confirmed COVID-19 infection during the radiotherapy or chemoradiation in April 2020 in National Cancer Research Center in Serbia. Data were obtained from the medical records between 1 April and 1 July 2020. Results: A total of 49 COVID-19 infected cancer inpatients were included. The most frequently reported cancers were head and neck cancers, in twenty-three patients (46.8%). Lymphopenia was present in 77.5% of the patients. Red blood cells, haemoglobin and platelets were significantly lower during incubation or diagnosis of COVID-19. Twenty-seven (55.1%) patients did not finish radiotherapy. The age of patients who finished radiotherapy after COVID-19 infection was significantly lower compared to the patients who did not finish radiotherapy (60.5 ± 7.8 vs. 68.6 ± 11.2; p < 0.005). Conclusions: COVID-19 infected cancer patients in radiotherapy practice show similar symptoms and demographic characteristics as the general population infected with SARS-CoV-2 virus. Patients with head and neck cancers may be susceptible to infection with COVID-19. Old age and male gender may be risk factors for discontinuation of radiotherapy in COVID-19 infected cancer patients.