Hansen-Aaroen is regarded as one of the top young talents in Norway and has been on trial with Everton and Liverpool recently, but a test spell with the Red Devils at their Carrington training base convinced the top six side to snap up the 14-year-old.
United had been in talks with Tromso for the Under 15s international for several months, and are said to have agreed a £90,000 deal that will see the 3-cap international move to the Premier League giant next summer when he turns 16.
The promising wonderkid cannot move to England immediately due to FIFA rules on the movement of youngsters between countries, but the deal has seen United part company with Aidan Barlow, who joins Tromso on loan for the season.
Hansen-Aaroen is a forward by trade and has scored one goal for Norway’s U15s following his debut in April.
Manchester United have also agreed a partnership with Tromso that may see more of their rising stars move to Old Trafford in the future, and Red Devils prospects going the other way in a bid to further their development.
It is not noted whether Everton officially approached Tromso to strike a deal for Hansen-Aaroen following his trial with the club.