The main aim of this work is to study the oxygen isotope composition of Late Pleistocene syngenetic repeated ice wedges and to obtain a high-resolution isotope diagram based on detailed sampling of the Seyakha yedoma section in the sequence of the Late Pleistocene third lagoonal-marine terrace near the mouth of the Seyakha (Zelenaya) River, on the shore of the Gulf of Ob. According to three new radiocarbon ages (25.2 to 23.3 ka BP) from absolute heights of +2 to +5 m and also ages obtained earlier (17.3 ka BP from +18.8 m; 11.6 ka BP from +21.2 m), the approximate age of this ice wedge ranges from 23 to 15–18 ka. The values of δ 18 О in the upper fragment of the ice wedge vary from –25.75 to –23.15‰. The amplitude of horizontal variations in δ 18 О at +15.2 m height is 1.49‰ (–23.41 to –24.9‰). The lower fragment of the ice wedge at +6 m height is characterized by a broader variation range of δ 18 О compared to the upper one: δ 18 О varies in the horizontal direction from –23.41 to –26.63‰. The detalization step of the oxygen isotope diagram obtained is 80–100 years. The mean January temperature in the period 23–15 (18) ka BP ranged from –35 to –39°C.