As many as 48% of cancer patients pursue popular diets, including the alkaline, Paleolithic, ketogenic, vegan, and macrobiotic diets, with the hope that they will improve survival and prevent recurrence. These diets have positive aspects consistent with the dietary guidelines proposed by the American Cancer Society (ACS) and the American Institute for Cancer Research (AICR). All of the diets emphasize eating vegetables, all but the ketogenic diet encourage eating fruit, and all but the vegan diet limit refined grains and alcohol. Both the macrobiotic and the alkaline diets meet the majority of the ACS and AICR guidelines. Negative aspects of these diets include pseudo-scientific rationales for their anti-cancer properties, limited evidence that they improve cancer outcomes, the possibility for nutrient insufficiencies, and elimination of food groups proven beneficial for cancer prevention and general health. Moreover, with nutritional counseling, nutrient insufficiencies and misalignment with cancer clinical guidelines can often be addressed. Clinicians should consider strategies to encourage evidence-based dietary changes that encourage positive features of popular cancer diets, while minimizing negative aspects.