Boro have seen off strong competition to complete the signing of highly-rated Dutch defender Rav van den Berg.
The 18-year-old joins from newly-promoted Eredivisie side PEC Zwolle on an four-year contract for an undisclosed fee.
Rav, brother of Liverpool defender Sepp van den Berg, enjoyed promotion in a breakthrough 2022/23 campaign with his boyhood club.
Boro rivalled some of the biggest clubs in Europe to secure the defender's services, with reports suggesting interest from the likes of AS Roma, Bayern Munich and AC Milan.
He becomes Boro's second addition of the summer window after the signing of forward Alex Gilbert.