Karanka was widely expected to succeed Tony Mowbray at the Riverside after Boro called a press conference for Wednesday morning.
And the north-east club said on their official Twitter feed: "Boro today announced that leading Spanish coach Aitor Karanka has been appointed as the club's new Head Coach."
The former Real Madrid defender brings a wealth of experience having worked under Jose Mourinho at the Bernabeu for three years from 2010.
Karanka left the club this summer following Mourinho's departure from the Spanish giants.
He had been linked with the vacant job at Barclays Premier League outfit Crystal Palace but has instead opted to move to join the Teesside club on a two-and-a-half-year contract.
Boro chief executive Neil Bausor said: "We are delighted that Aitor has decided to further his coaching career with Middlesbrough as our Head Coach.
"He brings with him some fantastic experience as a player and as a talented young coach. We are all excited about working with him."
Source: PA
Source: PA