Gallbladder agenesis is a congenital malformation that is considered extremely rare in dogs. The disease can course asymptomatically or with clinical signs, usually non-specific and including vomiting, anorexia, diarrhea, ascites, and lethargy. The objective of this report was to describe the clinical and anatomopathological aspects of a dog with hepatic encephalopathy secondary to gallbladder agenesis. This condition can be diagnosed during surgery or imaging examinations; however, it is often an incidental finding. In the biochemical examinations, a decrease in alanine aminotransferase and an increase in alkaline phosphatase and hypoalbuminemia were observed. During the necropsy, hepatomegaly was observed with absence of the gallbladder, congestion, cerebral edema, lipiduria, and pulmonary edema. Microscopically, there was intense fibrosis and inflammation in the liver due to chronic cholangiohepatitis (cirrhosis of the liver). The consequence of this lesion secondary to gallbladder agenesis was hepatic encephalopathy. Chronic liver failure exposes the cerebral cortex to toxins that are not metabolized by the liver, such as ammonia, mercaptans, short-chain fatty acids, scatols, indols, and aromatic amino acids. These toxins cause reversible damage to the brain, which results in neurological disorders. In this report, the dog had no clinical neurological signs, and the diagnosis of this condition was observed histologically. Dogs with gallbladder agenesis usually have clinical and pathological findings of hepatobiliary lesions such as cholestasis, cholangiohepatitis, and, in severe cases, hepatic encephalopathy, which are necessary to differentiate from other diseases that affect the hepatobiliary system, such as cholelithiasis, neoplasms, and chronic hepatitis.