Altteureu Airfield (Korean: 알뜨르 비행장), also known as Cheju-do No. 2 (K-40) Air Base, was a former Imperial Japanese Navy Air Service and United States Air Force (USAF) air base on southern Jeju Island, South Korea. It was mostly returned to farm land from the late 1960s onwards, though the site is still owned and used to some extent by the Republic of Korea Air Force, in particular a grass airstrip known semi-officially as Alddreu Airport.