The 37-year-old has seen himself sink down the pecking order at The Riverside, despite an excellent start to the season.
"I'm delighted that Coops will be joining us," said manager Mick McCarthy.
"I've known him for a long time; he's an experienced pro and will be a valuable addition to our squad for the run-in."
Cooper wants to continue playing for as long as possible and is believed to be looking to play for another season before hanging his boots up - and Boro are unlikely to give him that.
The Sedgefield-born star was in his second stint with Boro and has chalked up 398 appearances for the Teesside outfit - making him a real fans' favourite.
He did take on some coaching duties for Boro earlier in the season and that is almost certain to be an avenue he looks at in the future.
However, Cooper will now be looking towards The Black Cats and he is eligible for The FA Cup - which will be an added bonus for manager McCarthy.
"It's a fresh challenge for me," he said.
"The lads are playing well at the moments and I'll have to work hard to get in the team but it's great to get the opportunity to play football."
Meanwhile, McCarthy has confirmed that Sunderland do have an interest in Bradford's Simon Francis - who is also wanted by Sheffield United.
"We've been talking to the club and, yes, we've made a bid," said McCarthy.
"He's played regularly in the league this season and he's a good prospect. We've seen him, having played against him, but we've had him watched.
"He's got loads of potential to improve on what he has already achieved."
