The 23-year-old completed the formalities of his move across the Pennines on Wednesday and trained with the squad for the first time.
Karanka had said he wanted to add a midfielder during the transfer window, and Forshaw will go straight into his squad.
"He's just the type of player I want here," said Karanka. "He fits the profile of the players we want at the club.
"He's a good lad and he's a good player.
"It's important to bring in players who know the league and Adam fits that.
"He's a good addition to what we have here."
The midfielder was equally delighted to pen to paper on a three-and-a-half-year contract with the club.
"I was up for the challenge of coming to Middlesbrough," said Forshaw. "I know how big the club is. There's a good manager and a good set of players.
"Watching them from afar their style of play is good, it's something I wanted to be a part of, and I didn't need convincing."
You can see an exclusive interview with Adam Forshaw on Boro+ shortly and read more from him here.