Multiple diverticula of the right and left cardiac ventricular walls were incidental findings at necropsy in a juvenile, male, mixed-breed dog with no clinical history of heart disease. Each saccular diverticulum had a narrow communication into the corresponding ventricular chamber. Histologically, the diverticular walls consisted mainly of collagen without elastin fibers and atrophic cardiomyocytes. No inflammation or other lesions were associated with the diverticula or the rest of the myocardium. Based on the history and pathologic findings, a diagnosis of congenital biventricular fibrous diverticula was made.