Additional slips of the triceps brachii have been reported in cadaver studies. We report the case of a 48-year-old man with no history of trauma who presented with a tingling sensation and weakness in his right hand. Electromyography revealed ulnar neuropathy around the elbow, with decreased conduction proximal to the medial epicondyle. On ultrasonography and magnetic resonance imaging, the ulnar nerve was found to be mildly swollen and covered by an additional slip of the triceps brachii muscle above the retrocondylar area. Upon full elbow flexion, this anatomy produced dynamic compression of the vessels surrounding the nerve. Despite conservative treatments for over 2 months, the patient had minimal symptom improvement. Decompression and anterior transposition of the ulnar nerve were performed with favorable results. Additional slips of the triceps brachii muscle can compress neurovascular structures and cause ulnar neuropathy at the elbow. Ultrasonography is useful in the evaluation of such neurovascular compression.