Objectives. CT-based three-column classification (TCC) has been widely used in the treatment of tibial plateau fractures (TPFs). In its updated version (updated three-column concept, uTCC), a fracture morphology-based injury mechanism was proposed for effective treatment guidance. In this study, the injury mechanism of TPFs is further explained, and its inter- and intraobserver reliability is evaluated to perfect the uTCC. Methods. The radiological images of 90 consecutive TPF patients were collected. A total of 47 men (52.2%) and 43 women (47.8%) with a mean age of 49.8 years (. sd. 12.4; 17 to 77) were enrolled in our study. Among them, 57 fractures were on the left side (63.3%) and 33 were on the right side (36.7%); no bilateral fracture existed. Four observers were chosen to classify or estimate independently these randomized cases according to the Schatzker classification, TCC, and injury mechanism. With two rounds of evaluation, the kappa values were calculated to estimate the inter- and intrareliability. Results. The overall inter- and intraobserver agreements of the injury mechanism were substantial (κ. inter. = 0.699, κ. intra. = 0.749, respectively). The initial position and the force direction, which are two components of the injury mechanism, had substantial agreement for both inter-reliability or intrareliability. The inter- and intraobserver agreements were lower in high-energy fractures (Schatzker types IV to VI; κ. inter. = 0.605, κ. intra. = 0.721) compared with low-energy fractures (Schatzker types I to III; κ. inter. = 0.81, κ. intra. = 0.832). The inter- and intraobserver agreements were relatively higher in one-column fractures (κ. inter. = 0.759, κ. intra. = 0.801) compared with two-column and three-column fractures. Conclusion. The complete theory of injury mechanism of TPFs was first put forward to make the TCC consummate. It demonstrates substantial inter- and intraobserver agreement generally. Furthermore, the injury mechanism can be promoted clinically. Cite this article: B-B. Zhang, H. Sun, Y. Zhan, Q-F. He, Y. Zhu, Y-K. Wang, C-F. Luo. Reliability and repeatability of tibial plateau fracture assessment with an injury mechanism-based concept. Bone Joint Res 2019;8:357–366. DOI: 10.1302/2046-3758.88.BJR-2018-0331.R1