Increasing evidence indicates that obesity is a risk factor for various tumors. We aimed to clarify the evidence for an association between body mass index (BMI) and cancer risk based on existing systematic reviews and meta-analyses. Eighteen studies were included in this umbrella review after searching PubMed, Embase and Web of science. The results revealed that underweight was inversely associated with the incidence of brain tumors and positively related to the risk of esophageal and lung cancer. Overweight enhances the incidence of brain tumors, kidney cancer, endometrial cancer, ovarian cancer, multiple myeloma, bladder cancer and liver cancer. Obesity was related to the increased incidence of brain tumors, cervical cancer, kidney cancer, endometrial cancer, esophageal cancer, gastric cancer, ovarian cancer, multiple myeloma, gallbladder cancer, bladder cancer, colorectal cancer, liver cancer, thyroid cancer and Hodgkin's lymphoma. Moreover, dose-response analysis was conducted by 10 studies, and the results demonstrated that each 5 Kg/m2 increase in BMI was associated with a 1.01- to 1.13-fold increased risk of general brain tumors, multiple myeloma, bladder cancer, pancreatic cancer, breast cancer, and non-Hodgkin's lymphoma. Every 1 Kg/m2 increase in BMI was linked to 6% and 4% increases in the risk of kidney cancer and gallbladder cancer, respectively.