The dog in this case report had a retroperitoneal hemangiosarcoma with multifocal metastasis to the brain which was definitively diagnosed based on necropsy and subsequent histology. The patient presented to Cornell University Hospital for Animals (CUHA) with a one-day history of collapse, lethargy and inappetence. The patient was stabilized and treated for hypovolemic shock by the Emergency Service, due to a presumptive ruptured splenic mass. The following morning the patient was transferred to the Soft Tissue Surgery Service where further diagnostics such as abdominal ultrasound, revealed a large cavitated retroperitoneal mass. The owners elected to euthanize the patient due to the very poor prognosis and agreed to follow through with a teaching necropsy. The necropsy along with histology confirmed the diagnosis of a retroperitoneal hemangiosarcoma with multifocal metastasis to the brain. This paper will focus on the discussion of retroperitoneal hemangiosarcoma in the canine patient.