Mount Sidley is the highest dormant volcano in Antarctica, a member of the Volcanic Seven Summits. It is a massive, mainly snow-covered shield volcano which is the highest of the five volcanic mountains that comprise the Executive Committee Range of Marie Byrd Land. The feature is marked by a 5 km wide caldera on the southern side and stands NE of Mount Waesche in the southern part of the range.

The mountain was discovered by Rear Admiral Richard E. Byrd on an airplane flight, November 18, 1934, and named by him for Mabelle E. Sidley, the daughter of William Horlick who was a contributor to the 1933–35 Byrd Antarctic Expedition. The first recorded ascent of Mount Sidley was by New Zealander Bill Atkinson on January 11, 1990.