A 16-year-old neutered, male poodle presented with a large, hard mass in the dorsal perineal area. Computed tomography revealed a massive soft-tissue attenuated mass with multiple calcifications located in the anus, compressing the rectum. The huge mass, originating from the anal sphincter and distal part of the rectum, was completely surgically removed. Immunohistochemistry revealed that neoplastic cells were positive for alpha-smooth muscle actin and negative for c-Kit. Histopathology and immunohistochemistry confirmed well-differentiated leiomyosarcoma originating from the anal sphincter. The patient has shown a good treatment outcome and prognosis, with no significant postoperative complications or recurrence since the surgery during the 1-year follow-up period to date. To our knowledge, this is the first published report of a canine anorectal leiomyosarcoma that includes a computed tomographic investigation. CT imaging enabled successful preoperative assessment of the size, extent, and metastatic status of the anal mass, proving valuable for surgical planning and prognosis evaluation.