Dorsal dislocation of the proximal interphalangeal joint is a common injury in the orthopedic department. In most cases, the joint is reduced simply by closed manipulation. However, in rare cases, the joint is not reducible by closed manipulation, therefore, surgery is required. We report on a case of irreducible open dorsal dislocation of the proximal interphalangeal joint which was surgically treated. Because the flexor tendon interposed between the head of the proximal phalanx and the base of the middle phalanx, we could reduce the joint only after repositioning of the flexor tendon.