The 41-year-old took over at the Riverside in November 2013 after spending three years as Jose Mourinho’s assistant at Real Madrid.
Speaking to the club’s official website, he said: “When you are in the place where you want to be and you feel that the people like you, the club is the best club for me. I always said I couldn’t be in a better place than I am here, so it
was easy for me.
“I was speaking with the chairman and it took us 30 seconds. He knows me and I know him, and I know that I am in the right place with the right people at the right moment.
“After the (play-off) final there were some rumours from other clubs, but he (Steve Gibson) knows me and I’d never consider other things than to be here again.
“My relationship with Steve and the people at the club is more than the paper. It’s not about years or the money, it’s because we have an amazing relationship and I’m at the best club for me.
“For me, now I would like to be in the Premier League because I’m very passionate and like to do things as soon as possible.”
In a statement, Middlesbrough chairman Steve Gibson said: “This is a massive statement of the commitment Aitor has to this club.
“For me, this is probably the most important signing we will make all summer. We know that there has been interest in Aitor over the past year or so from clubs both here and abroad and rightly so because he is a very, very good manager.
“When that speculation has arisen, he has always been quick to say that he is happy here and working in the best place. Signing this new deal underlines that quite clearly. It gives us a lot of stability going into the new season and the coming years.”
Karanka, who previously enjoyed a playing career with Real Madrid and Athletic Bilbao, guided Boro to a 12th-place finish in his first season in charge after replacing Tony Mowbray.
He enjoyed a more impressive first full season at the helm, with Boro close to the automatic promotion places throughout the 2014/15 campaign before eventually losing in the play-off final to Norwich.
“All the players and the staff here think the world of him,” added Gibson.
“They admire and respect Aitor and the honest and principled way he goes about his business. He has to make some tough decisions at times but he always makes them fairly and with the good of the team in mind.
“Aitor is very happy here, his family is happy here and he is putting down roots. He is very quickly becoming a Teessider.
“He made a big decision to come here to another country and out of his comfort zone and has been overwhelmed by the welcome he has had from supporters and from people across the area. He has been touched.
“He understands the important role the team plays in the area and the passion people feel for Middlesbrough Football Club – and he shares that.”
Middlesbrough, who are the bookmakers’ favourites to win the title, kick off their season with a trip to newly-promoted Preston on Sunday.
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