We investigated the usefulness of ultrasound imaging and treatments for tenosynovitis of the extensor carpi ulnaris (ECU), based on clinical findings. Ultrasonography for diagnostic imaging of ECU tenosynovitis has not been previously reported. This study included 29 patients who were diagnosed as having ECU tendinitis and underwent ultrasonography. The short-axis view ultrasound images were characterized by an oval hyperechoic area in the ECU tendon from the distal to the peripheral end of the ulna on the affected side in all patients and a hypoechoic area suggesting a thickened synovial membrane of the tendon sheath around the ECU tendon. Patients with the most pronounced findings showed the greatest resistance to conservative therapy. Ultrasound images can be an indicator for differentiating ECU tendinitis from other diseases and determining therapeutic strategies. In cases with ulnar-sided wrist pain without these characteristic ultrasound imaging findings, injury of the triangular fibro cartilage complex should be suspected.