Cancer management in older patients has become an important public health concern due to the aging population and increased incidence of cancer with advanced age. The treatment decision for older individuals should not be constrained due to their advanced chronological age. Integration of an oncogeriatric approach is a major key point to improve treatment outcomes in older patients with cancer. Thus, comprehensive geriatric assessment should be used to evaluate older patients with cancer. Malnutrition (MN) is common in patients with cancer and has a profound effect on treatment outcomes. Therefore, assessing patients’ nutritional status and providing individualized nutritional intervention are essential. Radiotherapy (RT) is one of the most frequently used and effective treatment modalities against cancer in older adults and is a well-defined cause of MN. We herein highlight geriatric assessment requirements, including essential nutritional assessment in older patients with cancer. In addition to the general view, we focused on RT-related MN and its consequences. RT is better tolerated than surgery in older adults, but it may cause dehydration and MN due to RT-related diarrhea and mucositis. The clinicians should consider that in addition to its effect on the general clinical status, functionality, and surgical outcomes, the prognosis of RT is the worst in older individuals with MN.