The 29-year-old, who came through the Cardiff pathway and made 10 senior appearances before crossing the border, has established himself as one of the most consistent players in the English Premiership.
During a seven-season stay at Wasps, Young has made 141 appearances and scored 35 tries – more than any other forward in the Coventry-based club’s history.
His fine form for Wasps has led to three previous Wales caps, with eligibility rules preventing further appearances but Young has now taken the decision to return to his homeland.
He said: “I’m really excited to have this opportunity at Cardiff and I’m looking forward to returning home to Wales.
“It was a difficult decision to make because I’ve had such an enjoyable time at Wasps and learnt so much, which I remain very grateful for but I’m also a proud Welshman and the opportunity to stake a claim for further international honours was too great to turn down.
“Once I made that decision, Cardiff was always my preference – this is my home club. I am born and bred in the Cynon Valley, I played all of my age-grade rugby for the region and still have a lot of friends here from that time.
“I’m looking forward to linking up with the boys and contributing next season but remain fully committed to Wasps and want to sign off on a high.”
Young last featured for Wasps in Sunday’s Midlands derby victory over Northampton Saints, in which he was named man-of-the-match.
He has also captained the English giants on numerous occasions and Dai Young, who of course is the flanker’s father, believes he will be a significant addition for the Blue and Blacks.
The Cardiff Director of Rugby said: “When Thomas made the decision to return to Wales, we were of course very keen to bring him home to Cardiff – that goes for myself, the coaches and board.
“It is my job to improve and develop this squad and I have no doubt that Thomas will significantly add to our strength and depth in the back-row and drive standards in our environment.
“He suits the style we want to play and I know he will do his talking on the pitch and show everyone in Wales how good he is.
“This is just the first in a number of signings we will make in this building process and the journey we are on. I know the positions we want to strengthen and we will continue to do that.”
