Marten De Roon is Boro's latest summer signing after putting pen to paper on an four-year contract at Rockliffe Park on Monday.
The former Atalanta midfielder joins Viktor Fischer and Bernardo Espinosa as the club's latest signings ahead of the Premier League campaign.
Netherlands born De Roon, a 25-year-old defensive midfield player, joins from Italian Serie A side Atalanta with whom he earned rave reviews during his only season with the Bergamo side.
So far De Roon has made 204 club appearances, including four in the Europa League, scoring eight goals, having played with Sparta Rotterdam and Heerenveen before moving to Italy.
The transfer fee will remain undisclosed.