Purpose: Hypofractionated radiotherapy was recommended for several cancer sites to reduce outpatient visits during the coronavirus disease 2019 pandemic. This study aimed to identify the impact of the pandemic on hypofractionated radiotherapy for breast cancer in Japan. Methods and Materials: The monthly number of courses for hypofractionated and conventional radiotherapy was counted using sample datasets from the National Database of Health Insurance Claims and Specific Health Checkups of Japan, a nationwide database accumulating insurance claims data comprehensively. Changes in the number of hypofractionated and conventional radiotherapies were estimated using an interrupted time series analysis. Results: The number of hypofractionated radiotherapy courses gradually increased before the pandemic in contrast to that of conventional radiotherapy courses, which gradually decreased. However, conventional radiotherapy remained outnumbered by hypofractionated radiotherapy throughout the observation period. After the outbreak of the pandemic, the use of hypofractionated radiotherapy significantly increased in April 2020 (1,312 courses; 95% confidence interval: 801 to 1,823) but decreased in October 2020 (-601; 95% confidence interval: -1,111 to -92). Subgroup analysis by age and the number of beds in medical institutions revealed similar trends. Conclusions: Although conventional radiotherapy for breast cancer has been gradually replaced by hypofractionated radiotherapy, it remains predominant. The use of hypofractionated radiotherapy increased briefly early in the coronavirus disease 2019 pandemic; however, this increase was not sustained, unlike in other countries. Considering the benefits of hypofractionated radiotherapy for breast cancer, its use should be encouraged in Japan.