Muir Glacier, Valley glacier in Glacier Bay National Park, Alaska

Muir Glacier is a valley glacier in Glacier Bay National Park and Preserve, Alaska. Its terminus is about 0.7 km (0.43 mi) wide. In the mid-1980s it was a tidewater glacier that calved icebergs from a 90 m (200 ft) ice wall. Ocean water has since filled the valley, creating Muir Inlet.

Muir Glacier reached its Little Ice Age maximum at the mouth of Glacier Bay around 1780. In 1794 Captain George Vancouver found most of the bay covered by an ice sheet up to 1,200 m thick. From 1892 to about 1980 the glacier retreated nearly 32 km, and between 1941 and 2004 it lost over 12 km of length and more than 800 m of thickness. It is named after John Muir, who visited in 1879 and 1880 and published his account in Travels in Alaska in 1915.

John Muir's name was given to the glacier after his visits in 1879 and 1880, when he sent accounts of the area to the San Francisco Bulletin. He later gathered these pieces into Travels in Alaska, published in 1915, the year after his death.

You can reach the glacier by boat tours that depart from Juneau and travel north through Glacier Bay. The best time to visit is between May and September when waters are navigable and boat tours run regularly.

Muir Glacier was still a tidewater glacier in the mid-1980s, calving icebergs from an ice wall about 90 metres tall. By 2004 its terminus had moved inland and was no longer visible in the comparison photograph, and the Riggs Glacier had separated from it. Ocean water filled the vacated valley, forming Muir Inlet.