Septic peritonitis, defined as the presence of bacteria in the peritoneal cavity, is a life threatening condition that is frequently lethal. Patients with septic peritonitis present with non-specific signs and are frequently systemically ill with signs of shock. Abdominal fluid cytology is the diagnostic test of choice, though biochemical markers can be used as adjunct investigative tools. An immediate exploratory laparotomy is warranted when a diagnosis of septic peritonitis or pneumoperitoneum is made. Broad spectrum antimicrobials, abdominal drainage as needed, and aggressive supportive care must also be provided for the best chance of survival of the patient. In the following case, a 12 year old German Shepherd dog was referred to Cornell for treatment of an acute abdomen and pneumoperitoneum. Based on imaging and abdominal fluid cytology, the patient was determined to have a septic abdomen. This paper describes the pathophysiology, diagnosis, and treatment options for the condition.