Vertically unstable sacral fractures often make it difficult to achieve rigid fixation and there is no consensus on the optimal fixation technique for these injuries. The purpose of this study was to evaluate complication rate and short-term outcome of vertically unstable sacral fractures treated by posterior transiliac plate fixation. We performed a retrospective review of prospectively collected data of patients who underwent posterior transiliac plating for sacral fractures at two institutions. All patients were treated with the standard posterior approach using a 4.5-mm reconstruction plate and followed for at least 12 months. Patients' demographics, Majeed functional questionnaire surveys, and radiographic outcomes were collected. There were 19 patients with a mean age of 37.5-years. The mean follow-up was 26.3 months. The most frequent mechanism of injury was a fall from a height. According to the AO/OTA classification, there were 10 C1, 6 C2, and 3 C3, which were classified as 2 Denis I, 20 Denis II, and 2 Denis III, including 5 bilateral sacral fractures. Neurological deficit at the initial examination was recorded in 10 patients. The mean ISS was 20.7 and the mean timing of the internal fixation was 6.4 days. Anterior internal fixation of pelvic ring was added in eight patients. A Morel-Lavallee lesion was identified in 5 patients during the operation. Reductions were graded as nine excellent, seven good, and three fair according to the method of Tornetta. There were two postoperative surgical wound infections, both occurring in patients with a Morel-Lavallee lesion. All the sacral fractures united eventually and no implant failure occurred, though there were two patients with a small loss of reduction (<5mm) over the follow-up period. A total of 18 patients completed the functional assessment with a mean score of 78.5 points. Posterior plate fixation of vertically unstable sacral fractures is effective in maintaining fracture reduction even in the presence of significant posterior comminution. We caution its use in the presence of a known Morel-Lavallee lesion, as this may increase the wound complication and infection risk.