A 10-year-old castrated male Korean shorthair cat weighing 4 kg was referred with signs of insulin-resistant diabetes mellitus based on clinical signs of polyuria, polydipsia, and polyphagia. Diagnosis of pituitary-dependent hyperadrenocorticism (PDH) was made based on results of an adrenocorticotropic hormone stimulation test and a dexamethasone screening test. In addition, plasma concentrations of insulin-like growth factor 1 (IGF-1) increased. Radiography, ultrasonography, and computed tomography (CT) revealed hepatomegaly, renomegaly, and adrenomegaly affecting both adrenal glands as well as multiple cysts in a generally enlarged pancreas. Magnetic resonance imaging (MRI) showed that the cat’s pituitary gland was enlarged. The pituitary gland had a predominantly unilateral extension to the left. The signal intensity of the pituitary gland on precontrast T1 weighted images was hypointense compared to that of soft tissue and hyperintense compared to that of cerebrospinal fluid. On T2 weighted images, the pituitary gland was predominantly hypointense with a hyperintense rim. Contrast enhancement of the pituitary gland was not evident, and a mild degree of ring-like enhancement was seen. In addition, mild peritumoral edema was present. This is the first report of a cat with suspected double adenoma of the pituitary gland on the basis of compatible clinical signs, increased serum IGF-1 concentration, PDH, CT images, and MRI findings in diabetic cats with insulin resistance.