Climatic and sea level changes have influenced the evolution of estuarine and coastal environments during the Late Pleistocene and Holocene. These sensitive systems respond rapidly to eustatic, climatic, and anthropogenic forcing factors, and their responses are stored in sediments that can be analyzed using indirect indicators or proxies, such as diatoms. This study focuses on characterizing the environmental and climatic changes of a temperate coast in the Southern Hemisphere that has not been significantly affected by tectonic activity. It also aims to reconstruct micro and mesotidal coastal systems in relation to sea level changes in the southern Pampas. To assess past environmental conditions, diatom assemblages were analyzed from outcropping profiles and cores in the coastal plains and river basins of Buenos Aires province. The life forms and salinity tolerance of diatoms were used to distinghish marine, brackish, and freshwater sedimentary deposits from the Late Pleistocene to Holocene. These sedimentary patterns were used to plot Holocene sea-level fluctuations along the Pampas coast. The studied sequences have chronological control, providing essential information for reconstructing past environmental variability. The proposed sea-level curves for the region indicate a late Pleistocene sea-level low- and a mid-Holocene highstand. The identification of assemblages characterizing marine environments, tidal channels, coastal lagoons, and marshes allowed for the adjustment of diatom data to sea-level curves established for the region. The maximum sea level—between 6500 and 5500 14C years BP—was recorded in sediments containing diatoms typical of coastal marine environments. A diatom-based salinity transfer function was applied to twelve sedimentary sequences, enabling quantitative inferences about past salinity fluctuations along the Pampa coast during the Holocene.