A 12-year-old female neutered cockapoo was presented for further investigation of a suspected abdominal mass. Physical examination revealed a palpable abdominal mass and mild abdominal discomfort. No other clinical signs were reported. Haematological and coagulation factor analysis was normal. Serum biochemical analysis revealed a mild increase in alkaline phosphatase. Abdominal radiographs were acquired. A large, clearly marginated soft tissue mass was identified in the mid-abdomen. A large, non-specific mass originating from the body of the spleen was also suspected. A computed tomographic study of the thorax and abdomen revealed a large, clearly marginated, spherical mass of unknown origin within the mid-abdomen. A peritoneal cyst was suspected, and an exploratory laparotomy was performed. A large, fluid-filled, well-encapsulated peritoneal mass was removed from the mid-abdomen. Histological examination of the mass was consistent with severe chronic steatitis compatible with an omental fat abscess.