In dogs, splenic masses are common and possibly life-threatening. Although there is information available about long-term survival in dogs undergoing splenic surgery, less is available regarding short-term survival. In this retrospective study, the medical records of 539 dogs were reviewed. All had undergone splenectomy for known splenic mass at a large academic referral hospital. The most common malignant tumor was hemangiosarcoma (n = 228); the most common non-neoplastic lesion was hematoma (n = 104). Perioperative mortality rate was 41/539 (7.6%). Twenty-one dogs died following cardiopulmonary arrest, and 20 were euthanized because they were considered moribund. Causes of death included uncontrollable hemorrhage (n = 10), known or suspected portal system thrombosis (n = 9), suspected pulmonary thromboembolism (n = 4), known or suspected pneumonia (n = 4), and known or suspected disseminated intravascular coagulation (n = 3). For each decrease in platelet count of 10,000 platelets/µl at admission, odds of death increased 6%. Platelet counts may serve as a valuable gauge of overall coagulation status in smaller practices where coagulation testing may not be readily available. Dogs with a PCV <30% or dogs that developed cardiac arrhythmia during surgery were twice as likely to die as dogs with a PCV >30% or dogs that did not develop arrhythmia. Results indicate that reductions in perioperative mortality rate could potentially be achieved through improved recognition and treatment of hemorrhage and thrombotic and coagulopathic syndromes.