The oceanographic dynamics on the continental shelf off southeastern South America are primarily controlled by the southward-flowing warm Brazil Current, converging with the northward-directed cold Malvinas (Falkland) Current, and interacting with the continental discharge of the Plata River. The seasonally reversing regional wind field together with the seasonal cycle of riverine discharge, determines which of these three components provides the dominant forcing. The Uruguayan shelf is thus located in a transitional zone that extends from the region influenced by the Brazil-Malvinas Confluence (BMC) in the open ocean to the Subtropical Shelf Front (STSF) on the continental shelf. Understanding how the resulting oceanic seasonal variability responded to different climatic boundary conditions may shed light on its future behavior. This study presents the first reconstruction of mid-deglacial seasonal hydrographic variability on the continental shelf off southeastern South America in seasonal resolution based on stable oxygen and carbon isotopes (δ18O, δ13C) from a thick-walled shell of a long-living bivalve. The mid-deglacial (14.3 cal ka BP; Bølling-Allerød interstadial) Retrotapes exalbidus bivalve shows a mean δ18O of 3.27 ± 0.42‰ (2.50 ± 0.42‰ when corrected for changes in global ice volume) and a seasonal δ18O amplitude of 1.69‰ for raw isotopic excursions. Moreover, the δ13C exhibits abrupt negative peaks coincident with more negative δ18O values that indicate seasons of elevated freshwater discharge. Finally, the growth rate of the bivalve suggests that the specimen was closer to the metabolically optimum than modern individual of this species from southern South America. Combining biogeographic and ecologic information with these isotopic data, the results point to colder waters and a slightly lower mid-deglacial seasonal amplitude in temperature compared with modern conditions at this shelf site. Because of the northward-displaced Plata River mouth during deglacial times, negative δ13C peaks are expected to reflect an influence of non-point freshwater sources in the form of small fluvial distributaries along the paleo-coast. Most of this signal may, however, be driven by seasonal metabolic effects associated with low ambient water temperatures related to a shallow-water environment located closer to the respective paleo-coastline due to the low sea level at those times.