The purpose of this study was to evaluate dislocation of the extensor carpi ulnaris (ECU) tendon in patients with rheumatoid arthritis (RA) using three-dimensional computed tomography (3DCT). We then determined the value of 3DCT for predicting spontaneous extensor tendon rupture. 3DCT images of 102 wrists from 96 patients with RA were analyzed. Dislocation of the ECU tendon was evaluated from the 3DCT images with a soft tissue window. Dorsal subluxation of the ulnar head was evaluated as the dorsal subluxation ratio (DSR), and carpal supination was evaluated as the carpal supination angle (CSA) from the 3DCT images with a bone tissue window. Extensor tendon rupture was found in 43 of 102 wrists (42 %). Dislocation of the ECU tendon was found in 35 of 102 wrists (34 %). Dislocation of the ECU tendon was a strong risk factor for extensor tendon rupture in the multivariable logistic regression analysis, with an odds ratio of 26 (95 % confidence interval 7-99, p < 0.001). The DSR (p = 0.029) and CSA (p = 0.035) were significantly larger in wrists with a dislocated ECU tendon. Dislocation of the ECU tendon was well depicted using 3DCT images. It was strongly associated with extensor tendon rupture in the rheumatoid wrist.