Provided are magnetic tunneling junction devices, memory devices including the magnetic tunneling junction devices, and methods of manufacturing the magnetic tunneling junction devices. The magnetic tunneling junction device includes a first magnetic layer; a second magnetic layer disposed to face the first magnetic layer; and a first oxide layer disposed between the first magnetic layer and the second magnetic layer and including a metal oxide, wherein the metal oxide of the first oxide layer has a stoichiometrically oxygen-deficient composition, and wherein the second magnetic layer includes a magnetic material doped with a metal element.