Konstantopoulos has signed a one-year deal at the Riverside Stadium, and Karanka has revealed that it is reward for the 35-year-old's hard work since his arrival at the Championship outfit.
The Spaniard told the club's official website: "When I came, Jason Steele was the number one keeper. We brought in Shay, but Dimi kept working 100%.
"Then we brought in Tomas (Mejias) and still Dimi kept working. He then got his chance, played very well and deserves this contract."
The Greek stopper has been a regular in the Middlesbrough starting lineup since March 8.