Marine sediments occupy a significant portion of the Quaternary succession in the West Siberian Arctic but, until recently, their ages and distributions have been strongly debated. Based on new geological, palaeontological and geochronological data obtained within the last decade, we hereby postulate the following: (i) Two different Pleistocene interglacial marine formations are exposed across the West Siberian Arctic. The older Kheta marine formation, sandwiched between two Middle Pleistocene till beds, yields optically stimulated luminescence (OSL) ages of c. 200 ka (MIS 7). The younger Karginsky/Payuta marine formation, overlying the Middle Pleistocene Taz till, contains a boreal mollusc fauna and has an OSL age of c. 115 ka (MIS 5e). These Middle and Upper Pleistocene marine beds have been correlated across a vast region between the Ob’ and Yenisey estuaries. (ii) The marine transgression during MIS 7 spread as far south into West Siberia as 65°N, almost 1000 km southwards from the present-day Kara Sea shoreline. Meanwhile, the smaller marine transgression during MIS 5e reached as far south as 68°N. (iii) Both marine formations contain a boreal molluscan fauna, which is not found in the modern Kara Sea but is characteristic of the North Atlantic. We propose that warmer Atlantic waters penetrated much further eastwards during both MIS 7 and MIS 5e than at present. The diagnostic extinct mollusc species Cyrtodaria jenisseae has only been found in the older Kheta marine formation (MIS 7).