BackgroundMultiple carpometacarpal fractures and dislocations are rare. This case report describes a novel multiple carpometacarpal injury, namely, ‘diagonal’ carpometacarpal joint fracture and dislocation.Case presentationA 39-year-old male general worker sustained a compression injury to his right hand in the dorsiflexion position. Radiography indicated a Bennett fracture, hamate fracture, and fracture at the base of the second metacarpal. Subsequent computed tomography and intraoperative examination confirmed an injury to the first to fourth carpometacarpal joint along a diagonal line. The normal anatomy of the patient’s hand was successfully restored via open reduction combined with Kirschner wire and steel plate fixation.ConclusionOur findings highlight the importance of taking the injury mechanism into account to avoid a missed diagnosis and to choose the best treatment approach. This is the first case of ‘diagonal’ carpometacarpal joint fracture and dislocation to be reported in the literature.