The former Real Madrid assistant coach replaces Tony Mowbray as the Championship side's manager, having agreed a two-and-a-half year contract at the Riverside Stadium
The former right-hand man to Jose Mourinho will begin work at the Riverside Stadium immediately after agreeing a two-and-a-half year deal at the club.
Karanka, who was Mourinho's No.2 in Spain, is Middlesbrough's first-ever foreign coach and is excited to be working at a club recommended to him by the Portuguese.
"It's a very important day for me - it's my new job," he enthused to reporters.
"I think it's the best thing in the world because I find a club like a family, with a big stadium, big supporters.
"It's my time. I have worked with Jose Mourinho and he was a great influence, I learned a lot from him. He's a person who has helped me for months I always speak with him and he told me to come here."
Middlesbrough's chief executive, Neil Bausor, told the club's official website: "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."
Karanka played for Athletic Bilbao, Real Madrid and Colorado Rapids in a career spanning 15 years before becoming Madrid's assistant in 2010.