Forensic dental autopsy is a fundamental procedure for the study of a death under judicial investigation in the search for postmortem (PM) information that can be compared with the antemortem (AM) data available from the cadaver. One of the major difficulties in taking postmortem evidence is the opening of the mouth, limited in many cases by the phenomenon of cadaveric rigidity or rigor mortis. The aim of this study was to evaluate the effectiveness of a new device designed to facilitate the opening of the dental arches in rigor mortis corpses. For this purpose, a sample of 30 corpses (22 men and 8 women; mean age 42.18 years and 38.75 years) from the Institute of Legal Medicine of the city of Lima (Peru) was analyzed. A forensic odontologist designed and manufactured the apparatus and carried out the sample analysis process. The initial and maximum measurements of oral cavity opening were recorded and a paired Student's t-test was applied to observe the difference between these initial and maximum measurements obtained (p < 0.05). The results showed that the average oral opening obtained with the new appliance was 43.83 ± 3.62 mm, with an average initial opening of 6.49 ± 3.04 mm (p < 0.001). The efficacy was similar in men and women (maximum opening M: 51.70 mm - F: 53.00 mm). In 29 of the 30 carcasses studied (96.7%) an opening greater than 40 mm was obtained, the latter being a minimum measurement sufficient to access the oral cavity. The efficacy of this new instrument for oral autopsy purposes is highlighted, since it does not cause destruction of the soft tissues and allows the correct manipulation of the oral structures present.