Recently, quadrilateral plate (QLP) fractures of acetabulum have attracted increasing attention. However, evidence for the appropriate classification of QLP fractures is still lacking, making it difficult to understand and manage these fractures. This study aimed to introduce a new classification for QLP fractures and evaluate its reproducibility.A series of 1101 consecutive patients with acetabular fractures from 8 level-I trauma centers were enrolled in this study. All patients underwent preoperative radiograph and computed tomography imaging. QLP fractures were identified and classified using the new and Judet-Letournel classification system. The inter- and intra-observer reliabilities (kappa coefficients, κ) of these two systems were investigated by 4 observers. Furthermore, surgical approaches and fixation methods for each fracture type are described.In total, 243 (243/1101, 22%) patients with QLP fractures were identified and included in this analysis. The mean κ value of the intra-observer reliability was 0.84 (range, 0.763-0.919) for the new classification, indicating excellent agreement, and the inter-observer reliability was 0.762 (range, 0.625-0.876), indicating substantial agreement. The values were 0.649 (range, 0.523-0.708) and 0.584 (0.497-0.646), respectively, according to the Judet-Letournel classification. Six cases (6/243, 2.5%) could not be classified using the Judet-Letournel classification. The selection of surgical approaches and fixation methods depends on the fracture type; however, an anterior intra-pelvic approach and buttressing fixation using the plate or screw are preferred for QLP fractures.This study presents a new classification for QLP fractures, showing higher intra- and inter-observer reliabilities than those obtained using the Judet-Letournel classification. This allowed us to obtain an in-depth and comprehensive understanding of QLP fractures. Additionally, the new classification might guide further studies on surgical strategies for QLP fractures.Level II.Copyright © 2023. Published by Elsevier Ltd.