"From young, it has always been my dream to represent Arsenal," said the 22 year old Nigeria international. "It feels so special. The first kit I had was of Arsenal and long may I continue to represent them."
Boss Unai Emery said: "I'm very pleased Alex has signed a new contract."
He added: "He is a great example of a player who can succeed from the academy through hard work and I look forward to working with him and our other talented young players in the squad."