A 5-year-old neutered male domestic medium-hair cat was referred to the internal medicine department with upper respiratory signs and limb pain. Computed tomography (CT) revealed diffuse osteosclerosis, showing severe osseous thickening of the nasal turbinates and near-occlusion of the nasal passages. No new or healed fractures were observed. Additional diagnostic tests revealed no infectious or neoplastic causes. The cat was treated supportively for pain and nasal inflammation. One year after the diagnosis, the cat was described as having an acceptable quality of life at home, despite ongoing clinical signs of nasal congestion. Chronic upper airway congestion is a common presenting complaint in feline patients. This report describes the imaging findings in a cat in which upper airway signs were due in part to systemic bone pathology. Previous reports suggest that the most common causes of chronic nasal congestion are acquired conditions with poor prognosis, such as lymphoplasmacytic rhinitis, neoplasia, or fungal infection. Diseases that cause diffuse osteosclerosis in cats are poorly understood. This report describes a case of diffuse osteosclerosis in a young cat with no other known comorbidities. The clinical picture is similar to descriptions of an osteosclerotic bone disorder in humans commonly referred to as autosomal dominant osteopetrosis type 1. Since no targeted treatments are available to reverse the osseous changes, the patient was treated supportively with pain management and corticosteroids. Recent developments in our understanding and treatment of similar human diseases may guide future advances in veterinary medicine.