Despite the development of radiotherapy machines and technologies, a proportion of patients suffer from radiation-induced lymphedema. Saireito (SRT) is a traditional Japanese herbal medicine that has been used for treating edema and inflammation in conditions such as nephritic disease. This study investigated the effect of SRT on lymphedema caused by radiotherapy. Four patients were treated with SRT at a dose of 9 g/day. The severity of lymphedema was evaluated using the Common Terminology Criteria for Adverse Events version 4 and Numerical Rating Scale before and after SRT treatment. After the treatment with SRT, 2 of 4 patients (50%) showed apparent improvement in lymphedema. One of the cases had difficulty in wearing the custom-made thermoplastic cast, but after SRT administration, he could wear the mask easily. One case decided to stop taking SRT 3 days after initiation because cough and fever appeared. In conclusion, it is important to control the side effects of radiotherapy, which leads to improved tumor control rates. Prospective randomized studies are necessary to confirm the findings of this case series study.