The evolution of coastal sand barriers is closely linked to sea-level fluctuations and sediment supply, which in turn are associated with climatic variations. Climate also exerts control over the development of penecontemporaneous or late-stage aeolian deposits that become superimposed on these barriers. This study examines the chronology and sedimentological features of Barrier III, a Pleistocene sandy coastal barrier with an aeolian cover, located in the northern sector of the coastal plain of Rio Grande do Sul, Brazil. The stratigraphic succession observed in three vertical sections revealed two principal depositional sequences. Depositional Sequence 1 (DS1), attributed to MIS 5e, records the superposition of shallow-marine deposits and penecontemporaneous coastal dunes formed during the transgressive-highstand interval, reflecting barrier construction under relative sea-level rise. Depositional Sequence 2 (DS2) consists of late-stage aeolian deposits that reworked antecedent DS1 dune sands under lower sea-level conditions than present. The OSL ages obtained for DS1 range from 167 f 11 ka to 149 f 11 ka, which are older than typically expected for MIS 5e and are interpreted as overestimated, likely due to signal saturation, enhanced cosmic-ray contribution in a lowradioactivity setting, and/or incomplete bleaching. DS2 yielded ages between 61 f 5 ka and 141 f 0.16 years, indicating multiple phases of Late Pleistocene to Holocene aeolian activity. Aeolian sedimentation was thus recurrent throughout the barrier evolution, but controlled by different mechanisms: in DS1, dune formation was linked to shoreline position during the MIS 5e highstand, whereas in DS2, aeolian reworking was predominantly driven by climatic variability, independent of relative sea-level oscillations. Grain-size distribution and heavy mineral data show that the aeolian sands of DS1 inherited their heavy-mineral signature from underlying shallow-marine deposits, whereas DS2 deposits represent the short-distance reworking of DS1 dunes under renewed aeolian activity and pedogenesis. These findings underscore the importance of integrating sedimentological and chronological data to reconstruct the evolutionary history of coastal systems, offering insights into how ancient sandy coastal barriers responded both to sea-level changes and to climatic forcing.