Malltraeth Marsh (alternate: Malldraeth; Welsh: Cors Ddyga or Cors Degai) is a large marsh area in Anglesey, North Wales, located northeast of Malltraeth village, north of Llangaffo and south of Rhostrehwfa. It is reclaimed from estuarine marshes after the construction of the Malltraeth Cob (dyke), a long embankment, and the subsequent canalisation in 1824 of the Afon Cefni.