An 11-year-old, intact female Pekingese dog was presented because of chronic vomiting. Clinical examination and diagnostic imaging suggested the presence of an intestinal mass. Laparotomy was performed, and a gelatinous effusion associated with a suspected jejunal neoplasm was found. Accumulation of gelatinous material was grossly evident in several abdominal organs and parietal peritoneum. Cytopathological smears from this material showed macrophages, reactive mesothelial cells, and spindle cells embedded in a mucinous basophilic background. After spontaneous death, necropsy and histopathology were performed and showed the presence of an invasive mucinous adenocarcinoma with visceral and peritoneal metastases. The clinicopathological findings of this case report closely resemble those reported in human cases of pseudomyxoma peritonei.