Byrdbreen is, at about 40 nautical miles (74 km) long and 11 nautical miles (20 km) wide, the largest glacier flowing northwest between Mount Bergersen and Balchen Mountain in the Sør Rondane Mountains. It was mapped by Norwegian cartographers in 1957 from air photos taken by U.S. Navy Operation Highjump, 1946–47, and named for Rear Admiral Richard E. Byrd, U.S. Navy, commander of U.S. Navy Operation Highjump.