The present study was aimed to evaluate the safety, tolerability, and beneficial effects of a ketogenic diet (KD) on body composition and blood parameters and survival in patients with breast cancer. In this randomized, controlled trial, 60 patients with locally advanced or metastatic breast cancer and planned chemotherapy, were randomly assigned to a group receiving KDs (n = 30) or to a control group with standard diet (n = 30) for 3 months. Serum biochemical parameters and body composition were analyzed at baseline, every 3 weeks and end of each arm. Compliance and safety of KD were also checked weekly. Fasting blood sugar (FBS) was significantly decreased in intervention group compared to the baseline (84.5 ± 11.3 vs. 100.4 ± 11.8, P = 0.001). A significant inter-group difference was also observed for FBS level at end of intervention. There was an increasing trend in serum levels of ketone bodies in intervention group (0.007-0.92, P < 0.001). Compared to the control group, BMI, body weight, and fat% were significantly decreased in intervention group in last visit (P < 0.001). No severe adverse side effect was found regarding lipid profile and kidney or liver marker. Overall survival was higher in KD group compared to the control group in neoadjuvant patients (P = 0.04). Our results suggested that chemotherapy combined with KDs can improve the biochemical parameters, body composition, and overall survival with no substantial side effects in patients with breast cancer.