Summary This report describes a case of osteoma cutis diagnosed by physical examination, surgery and histopathological examination. A 2‐year‐old Thoroughbred colt with hindlimb lameness was brought to our facility. Physical examination showed a well‐circumscribed, large plate‐like mass covered with normal haired skin in the right thigh region. Ultrasonography revealed a subcutaneous linear mass. After surgical removal, histopathological examination showed thin mature bone surrounded by fibrocollagenous tissues. To the authors' knowledge, this is the first report of spontaneous idiopathic osteoma cutis in horses.