Abstract <p>The results of sediments study in Lake Pertozero in the south-western coastal part of the Onega Peninsula, complemented by previously published results of the studies of the outcrops of coastal terraces, enabled reconstructing the relative sea-level changes in the southern part of the Onega Bay in the Late Glacial and the Holocene. In the Late Glacial, the depression of Lake Pertozero was inundated by the waters of a freshwater proglacial lake, which level could reach 30 m a.s.l. The regression of the proglacial lake below the elevation of Lake Pertozero (11.6 m a.s.l.) took place around 11.1 ka cal. BP. After ~11 ka cal. BP and at least until ~10.1–9.5 ka cal. BP the relative sea level in the southern part of the Onega Bay exceeded ~4–5 m a.s.l. However, it did not reach the elevation of Lake Pertozero where freshwater benthic diatoms characteristic of recently isolated lakes flourished that time. The subsequent regression terminated prior to ~7.6 ka cal. BP. The relative sea level during the subsequent transgressive phase exceeded 7–7.5 m above sea level, however, remained below 11 m, as evidenced by the presence of exclusively freshwater species in the diatom assemblages of Lake Pertozero. The reconstructed dynamics of the relative sea-level changes in the southern part of the Onega Bay generally corresponds to those in the western part of the White Sea, the north-western and eastern coasts of the Onega Peninsula, however, the time frames and amplitudes of the transgressive and regressive stages differ markedly.</p>