The incidence of obesity in pets appears to be increasing in line with the increasing incidence of obesity in humans, and leads to decreased life expectancy. Obesity, which is considered a multifactorial disease caused by excessive adiposity, leads to a decrease in quality of life and serious health problems. It is known that there is an increase in the incidence of respiratory disorders, cardiological disorders, metabolic and endocrine problems, orthopedic diseases and some types of cancer in obese cats and dogs. There are many factors in the formation of obesity. One of these factors is the balance of the microbiota in gut. Many studies have shown that the microbiota affects critical steps in the formation of obesity and there are strong relationships between dietary content, microbiota, and obesity. In particular, high-fat diets are known to increase microbiome composition in terms of gram-negative bacterial strains and trigger dysbiosis. Again, in cases where dysbiosis occurs, the levels of volatile fatty acids also vary and lead to undesirable results through hormonal mechanisms. This condition, which causes hyperphagia, hypertriglyceridemia and insulin resistance, increases the incidence of obesity and diabetes mellitus. The ratio of Firmicutes and Bacteroidetes, which are among the largest phylae of the microbiota, shows serious differences when compared in underweight and obese animals. In this article, these relationships between microbiota and obesity are reviewed.