The Tatuí Formation records one of the most complex episodes of paleoenvironmental evolution in the Paraná Basin during the Early Permian, representing the transition between the glacial and periglacial conditions inherited from the Itararé Group and the establishment of a broad coastal system with a continental predominance. This evolution resulted from the interaction among fluvial, deltaic, lacustrine, lagoonal, and marine processes, developed over a paleotopographic surface inherited from the Late Paleozoic glaciation, whose compartmentalization into topographic lows and high areas controlled the significant faciological heterogeneity of the unit. In the depressions, fine-grained deposits associated with low-energy lacustrine and lagoonal environments predominated, whereas the elevated compartments acted as sediment source areas, giving rise to higher-competence fluvial systems such as the Ibicatu Paleoriver, a high-energy braided system responsible for the deposition of the Ibicatu Facies and the sediment supply to a broad progradational deltaic system. Low-energy environments, including ephemeral lakes, floodplains, abandoned paleochannels, and brackish lagoons influenced by incursions of the Rio Bonito–Palermo Sea, record periods of greater hydrodynamic stability, as evidenced by tidal structures and wave- and storm-generated reworking. Recurrent relative sea-level fluctuations promoted the alternation between continental and shallow-marine conditions, which is also reflected in the paleontological record. Siltstone deposits preserve conchostracans, crustaceans, and actinopterygian fish characteristic of temporary continental environments, while ichnofossils, acritarchs, and agglutinated foraminifera document episodes of marine connectivity. The Ibicatu Flora, represented by silicified trunks of Tietea and gymnosperms, demonstrates the presence of forested areas adjacent to the river channels. Taken together, these records indicate that the Tatuí Formation constitutes a transitional depositional system between continental and shallow-marine environments, whose evolution was controlled by the paleotopography inherited from the Late Paleozoic glaciation, climatic seasonality, and fluctuations of the Rio Bonito–Palermo Sea, configuring an important archive for the paleoenvironmental and paleoclimatic reconstruction of the eastern margin of the Paraná Basin.