In canines, when a mass in the bladder is observed by ultrasonography, a malignant tumor is a presumptive diagnosis. In humans, it is common in the presence of clots due to uncontrolled hematuria. We describe the case of a 7-year-old male mixed breed dog, with a clinical history of hematuria 6 months and presence of ticks. Ultrasound studies of the bladder were performed for one month. The evaluations showed a mass in the bladder trigone, covering 80% of the bladder lumen without acoustic shadow, and then a cystectomy was performed. An oval mass was found with irregular borders and clotted blood appearance, with not adherent to the bladder tissue. The histological diagnosis of the mass showed erythrocytes and polymorphonuclear inflammatory cells: neutrophils and mononuclear cells such as macrophages, mixed with moderate fibrin threads compatible with a bladder clot. Therefore, the diagnosis was a benign mass and not a transitional cell carcinoma as usual. Masses found in the bladder are not necessarily a neoplastic proliferation.