The ketogenic diet (KD) has attracted attention in recent years for its potential anticancer effects. KD is a dietary structure of high fat, moderate protein, and extremely low carbohydrate content. Originally introduced as a treatment for epilepsy, KD has been widely applied in weight loss programs and the management of metabolic diseases. Previous studies have shown that KD can potentially inhibit the growth and spread of cancer by limiting energy supply to tumor cells, thereby inhibiting tumor angiogenesis, reducing oxidative stress in normal cells, and affecting cancer cell signaling and other processes. Moreover, KD has been shown to influence T-cell-mediated immune responses and inflammation by modulating the gut microbiota, enhance the efficacy of standard cancer treatments, and mitigate the complications of chemotherapy. However, controversies and uncertainties remain regarding the specific mechanisms and clinical effects of KD as an adjunctive therapy for cancer. Therefore, this review summarizes the existing research and explores the intricate relationships between KD and cancer treatment.