The deal is worth 13.75 billion USD at catalogue prices and deliveries are due to start in 2018, Airbus said on the second day of the Farnborough airshow near London.
Plans for the neo, a new version of the long-standing A330 with upgraded Rolls-Royce engines, were launched by Airbus at the key industry event yesterday.
"AirAsia X, the long-haul affiliate of Asia's largest low-cost airline (Air Asia), has signed a memorandum of understanding with Airbus for 50 A330-900neo aircraft," said a statement.