The 35-year-old shot-stopper arrived on Teesside last summer after being released by Greek side AEK Athens, but had to wait until January to make his debut in an FA Cup clash with Hull City at the Riverside Stadium.
Since then, Konstantopoulos has gone on to make 12 starts in the Sky Bet Championship, displacing Jason Steele as the club's first-choice goalkeeper.
Boro boss Aitor Karanka revealed the former Hartlepool keeper's commitment since arriving was key to a new contract being offered.
"When I came, Jason Steele was the number one keeper," the former Real Madrid coach told the club's official website.
"We brought in Shay, but Dimi kept working 100 per cent.
"Then we brought in Tomasz and still Dimi kept working.
"He then got his chance, played very well and deserves this contract."