This paper presents new field and laboratory data combined with previously published materials on interglacial sediments underlying two upper tills of northern European Russia and West Siberia. The emphasis is on the structure and chronometric data of marine strata with shells of boreal molluscs of Cyrtodaria genus indicative of the Atlantic marine invasion into these sedimentary basins during the late Middle Pleistocene. Another line of evidence for this time span is inferred from inter-till terrestrial sequences in the European Northeast with forest pollen spectra suggesting a climate much warmer than today. The decisive correlation signal is provided by optically stimulated luminescence and electron-spin resonance dates, indicating MIS 7 time, and by the geological position of two interglacial formations between the first, second and third from the surface glacial complexes. The collected evidence disagrees with the model of the unique boreal transgression and confirms that saline Atlantic water at least twice invaded northern Russia during the last 250 kyears. The sedimentary formations of the penultimate interglaciation can serve as a new intra-regional stratigraphic marker for the northern Pleistocene.