Abstract Diffuse cutaneous mastocytosis was diagnosed in a 6-year-old, indoor, neutered female domestic European shorthair cat. Marked pruritus located mainly on the head and neck was noticed in the cat and in this area the animal had developed alopecia, crusts, and plaques. Histologically, monomorphic mast cells were found in the superficial dermis and around the hair follicles. Mast cells were well differentiated, with central nuclei and granular cytoplasm, with metachromatic granules which stained positively with Toluidine blue stain. The animal was successfully treated with oclacitinib at a dose of 1 mg/kg, twice a day per os.