Background Osteoid osteoma of the wrist is rare and treatment is still in debate. Case Description We report on a patient with osteoid osteoma of the head of the ulna, as an unusual cause of pain on the ulnar side of the wrist. The diagnostic delay was 4 years. A previous ulna shortening surgery was performed because of misdiagnosis. The diagnosis was made with bone scan and computed tomography (CT). A surgical resection was performed. The symptoms disappeared, and full range of motion was recovered. Literature Review Osteoid osteoma on the head of the ulna had never been described before. Clinical Relevance This case demonstrates that osteoid osteoma can mimic ulnocarpal impingement syndrome.