Radiolarian assemblages and geochemistry from Permian chert and siliceous mudstone in the Soi Dao, Chanthaburi area of southeastern Thailand, are used here to determine the age and depositional environment of these rocks. Moderate-to-well preserved radiolarian were obtained from three study areas, from bedded cherts and siliceous mudstones of Early Asselian to Early Capitanian, Sakmarian to Roadian, and Capitanian to Changhsingian age. Considering the lithofacies, age, and chemical composition of the rocks, a primary stratigraphy can be reconstructed that consists, in ascending order, of basaltic rocks, radiolarian bedded cherts, siliceous mudstones, and clastics made of alternations of sandstone and mudstone. Geochemical analyses, particularly chondrite-normalized REEs patterns of chert and siliceous mudstone, show a gradual change in the degree of the Ce-negative anomaly, which decrease towards the younger strata. This change indicates that the depositional environment of the Permian rock was at the beginning highly influenced by hydrothermal activity, which decreased with time, while the influx of terrestrial clastics increased. Permian bedded cherts accompanied by basalts and siliceous mudstones recognized in the study area are very similar to the Paleo-Tethys bedded cherts in terms of their lithofacies and microscopic features; however, their depositional period is much shorter, indicating that they were deposited in another oceanic basin (i.e., a back-arc basin). The chemical composition also shows that the influence of hydrothermal activity weakened over time and the influence of terrigenous clastics increased.