Ajayi, 17, was born in Hamburg, Germany, but his father is Nigerian, which means he qualifies for both nations, as well as England, as he climbs the ranks at United.
The teenager, who is a forward for the U18/U19, with U21s sides, has now put pen to paper, signing what the club have described as a "long-term" professional contract.
The club announced the news on Friday morning, revealing that they had locked down Ajayi for the foreseeable future.
“Exciting youngster Noah Ajayi has committed his future to Manchester United.
“The 17-year-old winger has signed a long-term professional contract.
“Ajayi was a stand-out for Darren Fletcher’s Under-18s side last season, making important contributions to help the young Reds reach the FA Youth Cup final. The youngster has also featured for United’s Under-21s.
“Everyone at Manchester United is delighted that Noah has signed a new deal and looks forward to seeing him progress.”
This news comes just one week after fellow academy prospect JJ Gabriel formally asked Man United to cancel his registration, a decision that the Red Devils rejected as they hope to cling on to the 15-year-old.
Real Madrid, Bayern Munich and Paris St-Germain are among the clubs reportedly keen on securing his signature and it will be interesting to see how United deal with the drama as the international break continues.