Obesity is a major modifiable risk factor for the development of numerous types of cancer. Although many factors contribute to obesity-driven tumorigenesis, this review focuses on the functioning of the gut microbiota (the microbiome) as an environmental risk factor for certain types of cancers, and presents possible biological mediators. Obesity is a well-studied condition that is associated with microbiotal dysbiosis, which could result in several physiologic changes that may contribute to the relationship between obesity and cancer risk. These include altered microbial metabolism, which contributes to the generation of procarcinogenic toxic metabolites; increased extraction of energy and nutrient availability leading to metabolic dysregulation that contributes to tumor growth; and/or the induction of subclinical inflammation initiating tumorigenesis. Thus, the gut microbiota may serve as a key link between obesity and cancer and, therefore, viable strategies to alter the microbiota may provide novel therapeutics to reduce obesity-associated cancer risk.