Armstrong has completed a permanent move to Championship side Blackburn.
Armstrong, who enjoyed a successful loan spell at Ewood Park last term, has joined Rovers for an undisclosed fee.
The 21-year-old came through the ranks at St. James' Park, making his debut for the Magpies as a substitute at Fulham in March 2014 at the age of just 17.
Armstrong – who lifted the Under-20 World Cup with England in the summer of 2017 – made a total of 21 appearances for United, while he also spent time out on loan at Coventry City, Barnsley, Bolton Wanderers and Blackburn in recent seasons.
He now returns to Tony Mowbray's side, who he helped win promotion from League One last term, on a permanent deal.