Armstrong has spent a large part of his footballing career at United, coming through the ranks of the club from a young age.
He made his first-team debut for the club back in March 2014 when he came on as a substitute against Fulham and only 17-years-old.
Whilst at the club he made 21 appearances, however, spent a large proportion of his time on loan to other clubs.
He spent time at Coventry City, Barnsley, Bolton Wanderers and last term, Blackburn.
Armstrong has taken his success from his loan spell at Tony Mowbray's side and decided to make a permanent transfer to the club that he helped secure promotion to the Championship from League One last season.
Rovers have secured the transfer for an undisclosed fee.
The 21-year-old has so far had a successful career, being part of the England Under-20's World Cup side who lifted the championship trophy in the summer of 2017.
Of the youngster's departure, Newcastle said: "The club wishes Adam well for the future."
Ex-United players have taken to social media to wish Armstrong the best including Alan Shearer who said: "Great news, good luck @AdamArma9"
Rob Lee also chimed in on the exit messages, tweeting: "Newcastle hardly have an abundance of goalscorers these days ......amazed @AdamArma9 never given a chance!!! Hope we don’t regret that !!"
Armstrong himself has not realised an official statement, however, he has responded to the kinds words and support from fans and ex-players who have not only thanked him for his time and role at Newcastle but who also wish him the best in the future and in his time at Blackburn.