Longstaff, who has been keen to leave the club on loan in order to play regular first team football, has signed a one-year extension to his Newcastle contract as part of the deal.
The England U20 international has fallen down the pecking order at his boyhood club – with the likes of Joe Willock, Jonjo Shelvey, Isaac Hayden and his brother, Sean Longstaff, all favoured ahead of him currently.
The 21-year-old – best known for scoring the winner on his Premier League debut against Manchester United in 2019 – links up with Stephen Glass' Dons side, where he is almost guaranteed to be a starter.
Aberdeen boss: 'We’ve no doubt he will have a positive impact'
After Longstaff completed his move, Aberdeen manager Glass said: “For a young player, Matty has already played in a good number of high-profile games for a top Club, against some of the biggest teams in England. We believe he is going to continue to make good progress here with us.
“We are pleased he has chosen Aberdeen to help him further his career, particularly as he had other high- profile options in England to consider.
“We’ve no doubt he will have a positive impact and will help us with the rest of our campaign.”
A move he needed at this point in his career
Many Newcastle fans will be disappointed to see Longstaff heading out on a season-long loan instead of fighting for his place in Steve Bruce's starting line-up – but it has been made evidently clear throughout the entirety of last year that the 21-year-old is not part of the club's imminent plans.
At this stage of his career, Longstaff has to be playing regular football and at Aberdeen, that is exactly what he will get. However, it is a surprise that he is not heading to play for a Championship club this season, as opposed to moving north to Scotland.
Playing in the second-tier in England this season potentially could have been the best for his development; something that has been emphasised by the emergence of the likes of Oliver Skipp, James Garner and Harvey Elliott, who all made big impressions on the Championship whilst on loan from Premier League clubs last season.
But if Longstaff can make an impression in the Scottish Premier League like Newcastle defender Paul Dummett did at St Mirren in 2012, it could be the making of his future career in black and white.