Placodonts are a group of turtle‐like animals, including placochelyids that are restricted entirely to the Triassic Tethyan realm. Their fossil remains are rare compared with other contemporaneous marine reptiles. Here, we report the first subadult specimen of the placochelyid Psephochelys polyosteoderma from south‐western China. It is the most complete specimen of this taxon known. The new fossil is referred confidently to Psephochelys polyosteoderma on the basis of the narrow and spatulate rostrum, toothless premaxillae, and carapace composed of small and numerous armour plates with relatively smooth surfaces. The new specimen provides additional anatomical information for Psephochelys polyosteoderma including the presence of transverse processes on the dorsal vertebrae lacking anteroposterior expansion, and a phalangeal formula in the pes of 1‐2‐3‐4‐2. The phylogeny of placodonts, especially the family Placochelyidae, is revised based on a character matrix incorporating additional postcranial characters. This resulted in a better‐resolved tree when compared with previous studies, with Psephochelys as a derived genus within Placochelyidae. On comparison with the adult holotype specimen, the incompletely developed armour in the new specimen reveals a fusion pattern of the carapace beginning with the axial and marginal regions. This surface fusion pattern resembles the ontogenetic process in the embryonic turtle shell, hinting at a possible deep homology with their common diapsid ancestor.