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, southwest of the Hindu holy site of Gaumukh (the source of the Bhagirathi River). It is the second highest peak on the south side of the Gangotri Glacier (after Kedarnath), 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 Jogen group of peaks, and has the lake Kedartal at its base.