Lewis Glacier is in the U.S. state of Oregon. The glacier is situated in the Cascade Range at an elevation generally above . Lewis Glacier is on the southeast slopes of South Sister, an inactive stratovolcano. Since its last maximum extent during the Little Ice Age (1350-1850 A.D.) where it descended to an elevation of , the glacier has been in a general state of retreat and the terminus of the glacier was reported in 2005 to be at approximately .