The 22-year-old, the son of former Boro midfielder Stuart, has committed his future to the club with Karanka having monitored the Academy graduate’s development.
The Spaniard told the club’s official website: “I’m really pleased with Connor because he has improved a lot since he came here.
“He is a very good lad. He’s from the Academy and he’s training hard and hopefully he can play more this season because he needs more experience.”
Ripley had made three senior appearances for the Teessiders and was a regular among the substitutes during the second half of last season.
The move to secure him comes just days after Boro missed out of experienced Republic of Ireland international Shay Given, who opted to join Barclays Premier League Stoke instead.
Ripley will once again compete with Dimi Konstantopoulos and Tomas Mejias for a place in the starting line-up.