Sagittal split osteotomy (SSO) is a maxillofacial surgery procedure that is used to correct mandibular prognathism, retrusion, or asymmetry. During an SSO, the use of sharp rotary tools for the osteotomy can induce complications, especially during the osteotomy of the medial side of the mandibular ramus. In this study, to decrease SSO complications, a computer-assisted, patient-specific sagittal split osteotomy guide and soft tissue retractor is developed. Computed tomography (CT) images of a human cadaveric mandible were digitally converted into a three-dimensional (3D) model. Then, a case-specific 3D model of the proposed device was designed for the surface of a cadaveric mandible, with the osteotomy line and geometric dimensions of the mandible both taken into consideration. The created 3D model was used to manufacture the device using the metal laser sintering method Finally, an SSO with the device was performed on the cadaveric mandible used to acquire the CT data. The proposed device ensures that the osteotomy planned in computer-aided preoperative preparations is applied correctly in the operation. Its use during an SSO is expected to shorten the operation duration and time needed for general anesthesia, resulting in less exposure time to bacteria. The shorter operation time is expected to reduce complications, the postoperative hospitalization period, and required corticosteroid amount for edema control.