United confirmed the agreement on Friday, with Ajayi rewarded after making significant progress through the club’s academy.
Born in Hamburg, Ajayi moved to England with his father when he was 10 and subsequently joined Manchester United’s youth system.
He signed a scholarship agreement with the club at the age of 16 before making the step up to a professional contract.
Ajayi has represented Germany at Under-18 and Under-19 level and has previously attracted interest from clubs in his country of birth, including FC Augsburg.
Manchester United have now tied the winger down as they continue to develop him within their academy structure.
The teenager enjoyed a productive campaign for the club’s youth teams and has been regarded as one of the emerging talents within United’s system.
His new contract represents another step in his development as he works towards eventually establishing himself in senior football.
United announced the agreement on their official channels, confirming that Ajayi will remain at Old Trafford as he continues his progression.