Abstract <p>In southwestern Primorye, the Upper Triassic is represented by the continental, often coal-bearing Mongugai Formation, which is part of the Laoeling–Grodekovo terrane, a fragment of the eastern edge of the Central Asian fold belt. The lithological composition of terrigenous rocks of the Mongugai Formation has been poorly studied; therefore, the aim of this article is to study in detail the geochemical composition of its sandy rocks and, based on genetic interpretation, to clarify geodynamic deposition environments and to determine the type and composition of rocks in source areas. As a result of the research, it was established that in their geochemical parameters, sandstones are homogeneous and are attributed to graywackes and, in part, lithic arenites. The rocks are rock-forming, having gone through one resedimentation cycle, and are characterized by a significant degree of chemical maturity of the clastic material, suggesting their formation due to the erosion of significantly weathered rocks of denudation areas. The specific features of the geochemical composition of sandy rocks, as well as the results of its paleogeodynamic interpretation, indicate that in the Late Triassic in southwestern Primorye, terrigenous deposits accumulated in an active continental margin setting, most likely complicated by strike-slip dislocations along transform faults. The main source area was a continental land composed of granite–metamorphic complexes and sedimentary rocks. Additional sources of clastics were granitoids of the root part of the deeply eroded ensialic arc, as well as the intermediate and basic volcanites from underlying Permian deposits.</p>