Radiotherapy is a common cancer treatment, and concurrent nutritional interventions can maintain nutritional status and improve clinical and supportive care outcomes. However, optimal nutritional interventions during radiotherapy are not firmly established. Herein, we assessed the feasibility, safety, and efficacy of dietary counseling interventions without oral nutrition supplements on health outcomes in adults receiving radiotherapy for cancer in a systematic review. Prospective clinical trials that implemented nutritional counseling interventions during radiotherapy were identified from four databases from inception through December 2023. Feasibility, safety, and efficacy were extracted from 32 articles that described 23 randomized and 4 non-randomized clinical trials. The interventions included individualized nutritional counseling (n = 14 articles), nutritional counseling plus exercise (n = 4), and nutritional counseling focused on increasing or reducing intake of specific nutrients (n = 9). Trials targeted head and neck (n = 12), pelvic cancers (n = 14), and/or breast (n = 5) cancers. Control groups had variable designs and included general nutrition education and intervention as needed. Studies recruited 120 ± 104 participants (range 26-468). Interventions tended to be feasible regarding retention and attendance at sessions, though feasibility metrics varied among different interventions. Most interventions were safe with no studies reporting adverse events attributable to dietary intervention. Individualized dietary counseling interventions tended to lead to between-group differences favoring the intervention group in regard to improved nutritional status, maintenance or attenuation of loss of body mass, improved quality of life, and reduced radiation-induced toxicities. Diets that encouraged/discouraged specific nutrients tended to recruit patients receiving radiation to the pelvic area and resulted in positive or neutral effects on gastrointestinal symptoms. In conclusion, nutritional interventions appear to be feasible, safe, and effective during radiotherapy for various symptom outcomes.