Thwaites Glacier (75°30′S 106°45′W / 75.500°S 106.750°W) is an unusually broad and fast Antarctic glacier flowing into Pine Island Bay, part of the Amundsen Sea, east of Mount Murphy, on the Walgreen Coast of Marie Byrd Land. Its surface speeds exceed 2 km/yr near its grounding line, and its fastest flowing grounded ice is centred between 50 and 100 km east of Mount Murphy. It was named by ACAN after Fredrik T. Thwaites, a glacial geologist, geomorphologist and professor emeritus at the University of Wisconsin–Madison. Thwaites Glacier drains into West Antarctica’s Amundsen Sea and is closely watched for its potential to raise sea levels.
Along with Pine Island Glacier, Thwaites Glacier has been described as part of the "weak underbelly" of the West Antarctic Ice Sheet, due to its apparent vulnerability to significant retreat. This hypothesis is based on both theoretical studies of the stability of marine ice sheets and recent observations of large changes on both of these glaciers. In recent years, the flow of both of these glaciers have accelerated, their surfaces lowered, and the grounding lines retreated.