The 25-year-old central midfielder joins Bernardo Espinosa and Viktor Fischer through the door at Rockliffe Park as Middlesbrough prepare for their first Premier League campaign in nearly a decade.
De Roon becomes Boro's second most expensive signing, behind Afonso Alves whom also played in Holland for Heerenveen.
De Roon spent three years at the Abe Lenstra Stadion after signing from Sparta Rotterdam on a free transfer.
After moving to Italy in 2015, he became a crucial first-team player for Atalanta and went on to win their Player of the Year award.
Where is the room for de Roon?
Despite Middlesbrough looking strong in the central midfield area during the 2015/16 season, manager Karanka has decided to splash a lot of cash to recruit in this area.
Captain Grant Leadbitter and Middlesbrough Supporters South's Player of the Year winner Adam Clayton occupied the two central defensive midfield roles in Karanka's favoured 4-2-3-1 formation as Boro secured a place in England's top tier. The arrival of de Roon would imply one of these two players would drop out of the starting eleven.
Also, Adam Forshaw has been linked away from the club in recent weeks and his departure looks to be edging ever closer, after the arrival of Julien de Sart in January and now with the de Roon signing.
Who will Boro sign next?
Local paper The Gazette reported today that Middlesbrough are hoping to sign Gaston Ramirez this week, should they recieve his work permit. After Ramirez's representatives cancelled his pre-contract agreement with Uruguayan club Penarol, he agreed a three-year deal with the Teesside club.
This, however, looks to be the only likely piece of transfer business for Middlesbrough. Interest in Spanish strikers Fernando Llorente and Lucas Perez seem to have cooled somewhat.
After losing out on Aissa Mandi and Thomas Meunier, to Real Betis and PSG respectively, Aitor Karanka is still on the lookout for a right back, but no signing has been made yet.