Taiwan, with its rich biodiversity since the Neogene period, has been the subject of extensive paleontological research; however, studies of its molluscan fossil fauna remain outdated. Contemporary gastropod diversity in the surrounding waters is considerably higher than that reflected in Quaternary deposits, suggesting that certain Pleistocene gastropod groups may have been undersampled or overlooked. To evaluate potential gaps in the Pleistocene mollusk record, we examined bulk sediment samples from two strata of the Toukoshan Formation (northern Taiwan) and the Szekou Formation (southern Taiwan). We focused on small gastropods (<5 mm), which may have been neglected in previous investigations. Specimens were sorted, identified to the lowest possible taxonomic level, and compared with both fossil and modern reference collections. We identified more than 150 taxa previously unreported from the Taiwanese Pleistocene record. These include representatives of Patellogastropoda; Vetigastropoda (≥4 families, 1 newly recorded family); Neomphaliones; Caenogastropoda (≥9 families, 5 newly recorded); and Heterobranchia (≥5 families, 4 newly recorded, and 1 newly recorded order). Several taxa represent their first occurrence as fossils, thereby extending their known geological ranges. Notable findings include pelagic Littorinimorpha and Pteropoda, as well as Cephalaspidea specimens potentially representing new species. Our results reveal a substantially richer and more diverse Pleistocene molluscan assemblage than previously recognized in Taiwan. These findings improve assessments of Late Pleistocene species richness, facilitate comparisons with the modern Indo–West Pacific fauna, and contribute to understanding the timing of lineage appearances in the region. The expanded fossil inventory will also enhance future paleoecological and biogeographical reconstructions, shedding light on the processes underlying the establishment of present-day Indo–West Pacific marine biodiversity.