Management of older cancer patients has become an important public health problem due to the aging population and increased cancer incidence with advanced age. The treatment decision of an older individual should not be constrained due to an advanced chronological age. The integration of the oncogeriatric approach is a major key point to improve outcomes in older cancer patients. Thus, older patients should be evaluated with a comprehensive geriatric assessment (CGA) during cancer care. Malnutrition (MN) is common in the cancer setting and has a profound effect on treatment outcomes. It is essential to assess the patients’ nutritional status and provide individualized nutritional intervention. Radiotherapy (RT) is one of the most frequently used and effective treatment modalities against cancer in older adults, and it is a well-defined cause of MN. Here, we draw attention to the requirement of geriatric assessment, including the essential nutritional assessment in older cancer patients. In addition to the general view, we focused on RT-related MN and its consequences. While RT is tolerated better than surgery in older adults, it may cause dehydration and MN due to RT-related diarrhea and mucositis. The clinicians should consider that, in addition to its effect on general clinical status, functionality, and surgical outcomes, the prognosis of RT is the worst in older individuals with MN as well.