An 8-year-old, female Pointer dog was presented with weight loss, hematochezia, and dyschezia. For the diagnosis, physical examination, complete blood counts, serum chemistry, radiography, ultrasonography, computed tomography, colonoscopy, cytology and histopathological examination were performed. The complete blood counts and serum biochemistry results were within the reference range. Ultrasonographic findings were presented with the thickened wall of the colon and rectum. Colonoscopy revealed irregular and ulcerated mucosal surface, mass and luminal narrowing of colorectal lesions. There were thickened wall and contrast enhancement of the lesion in colorectal region on the computed tomography. The cytologic examination suggested the adenocarcinoma. Postmortem histopathologic examination revealed adenocarcinoma. Based on these findings, the dog was diagnosed with annular colorectal adenocarcinoma.