Thalay Sagar is a mountain in the Gangotri Group of peaks in the western Garhwal Himalayas, on the main ridge that lies south of the Gangotri Glacier. It lies in the northern Indian state of Uttarakhand, 10 kilometres (6 mi) southwest of the Hindu holy site of Gaumukh. It is the second highest peak on the south side of the Gangotri Glacier, but it is more notable for being a dramatic rock peak, steep on all sides, and a famed prize for mountaineers. It is adjacent to the Jogin group of peaks, and has the lake Kedartal at its base.

Gangotri is located in Uttarkashi District, Uttarakhand, India in a region bordering Tibet. This glacier, one of the primary sources of the Ganges, has a volume of over 27 cubic kilometers. The glacier is about 30 kilometres long and 2 to 4 km wide. Around the glacier are the peaks of the Gangotri Group, including several peaks notable for extremely challenging climbing routes, such as Shivling, Thalay Sagar, Meru, and Bhagirathi III. It flows roughly northwest, originating in a cirque below Chaukhamba, the highest peak of the group.