Splenic masses, the majority of which are formed by hemangiosarcomas, are frequently observed in dogs of older age compared to other species and ages. A 11-year-old nonneutered male Beagle was admitted to the hospital with the complaints of abdominal distension with non-specific findings such as anorexia, stagnation, and weight loss. Severe abdominal distension with pallor of mucous membranes on clinical examinations; leukocytosis with anemia in the hemogram; high BUN and creatinine levels and a significant increase in liver enzymes in serum biochemistry were determined. In the microscopic examination of the aspirate taken by fine needle aspiration of the splenic mass which determined during ultrasonographic examination, a large number of pleomorphic, multinuclear neoplastic cells with eccentric nuclei, and different amounts of eosinophilic cytoplasm were detected. In this case report, in cases of limitations where biopsy or laparotomy could not be performed due to conditions such as the vascular structure of the mass or patient's disapproval; it was demonstrated that mass presence determined by ultrasonographic examination and the number of infiltrating mast cells determined by microscopic examination of the aspirate taken by fine needle aspiration can provide information in determining the benign or malignant character of the mass.