OBJECTIVE: To evaluate whether abdominal ultrasound correctly diagnosed septic peritonitis and correctly identified its causative lesion in dogs and cats. ANIMALS: 84 client-owned dogs and 10 cats that underwent an abdominal ultrasound and had confirmation of septic peritonitis via exploratory laparotomy or necropsy. METHODS: This retrospective case series documented abdominal ultrasound findings, surgical or necropsy findings, and method for initial diagnosis of septic peritonitis, if different from surgery or necropsy. The surgical report and necropsy findings were compared to sonography results to confirm a diagnosis of septic peritonitis. The frequency at which sonography diagnosed septic peritonitis and its causative lesion was calculated for each type of lesion pathology and organ system. Secondary aims included evaluating the effect of patient characteristics (body weight and species) on sonographic results and whether lesion type or location affected mortality. RESULTS: Most lesions causing septic peritonitis (70.2%) were gastrointestinal in origin and were nonneoplastic ulcerations or perforations (50%). Abdominal ultrasound diagnosed 56.3% of cases of subsequently confirmed septic peritonitis and correctly identified 67% of the causative lesions. Lesions of the gastrointestinal tract and ulcerations/perforations were the most frequent correct sonographic diagnoses and most likely to lead to a correct sonographic diagnosis of septic peritonitis. Lesions located in the hepatobiliary system and lesion types other than neoplasia or ulcerations/perforations were the most frequently missed by abdominal ultrasound. CLINICAL RELEVANCE: Abdominal ultrasound often fails to diagnose septic peritonitis or the underlying causative lesion, and its accuracy depends on the affected organ and type of lesion.