Mendelssohn Inlet () is an ice-filled inlet, long and wide, situated between Derocher Peninsula and Eroica Peninsula on the north side of Beethoven Peninsula, in the southwest part of Alexander Island, Antarctica. The inlet was first sighted from the air and roughly mapped by the United States Antarctic Service, 1939–41, and was resighted and photographed from the air by the Ronne Antarctic Research Expedition (RARE), 1947–48.