ABSTRACT - Adenomyosis, a non-neoplastic myometrial proliferation, is a rare pathologic condition in domestic animals, of unknown etiology. There is a lack of information about the clinical and pathological features of this disease in cats, therefore just a few reports in that species. The study describes a case of adenomyosis in a feline female. A cat, six years old, undefined breed, not spayed, had abdominal swelling history and vaginal discharge. Clinically, upon abdominal palpation, a diffuse increase suggesting a uterine change was found. The vulva had sanguine-purulent exudates. The choice was for the neutering surgical. Uterine macroscopy revealed cystic endometrial hyperplasia/pyometra in uterine horns, and the presence of a diffuse mass on uterine corpus. Fragments of the mass were sent for histopathological analysis, which revealed adenomyosis of the uterine corpus. The patient exhibited adequate postoperative recovery. The adenomyosis should be considered as a differential diagnosis of uterine neoformation in domestic cats.