Based on sedimentology, geochronology, palynology, and diatom analyses from core silt sediments in Cassino Beach (32°11′06″ S and 52°09′45″ W), southern Brazil, the Holocene marine transgressive stage was established. The absolute age of one sample is about 4940 ± 80 years BP. The palynomorphs (pollen and spores of vascular plants, zygospores and colonies of Chlorophyceae, cysts of dinoflagellates and acritarchs, fungal spores, and microforaminifera), silicoflagellates, and diatoms indicate the presence of an inlet bay in the southern part of the coastal plain during the marine transgression. The changes in the taxonomic composition, abundance, and frequency of palynomorphs and diatoms from the samples corresponding to transgression show an oscillatory character of the sea level. The posterior marine regression resulted in sand deposition and dune formation. The results demonstrate the importance of palynomorph and diatom application for the palaeoenvironmental reconstructions in coastal plains.