Thomas joins Wasps with a wealth of experience at the highest level having made 190 appearances in over a decade with Cardiff Blues.
The 29-year-old has also been capped 27 times for Wales since making his debut in 2005.
London Wasps director of rugby Dai Young said: "I obviously know Rhys very well having coached him for many years and he will be a real asset to our squad next season.
"He is an experienced international and exactly the kind of quality player we want to bring to the club. Rhys is also a great guy to have around a squad and he will be a great addition to the Club off the pitch as well."
Thomas added: "I am really looking forward to linking up with Dai Young again, who has been a big influence to me in my career and I'm excited by the challenge of playing in England with London Wasps.
"Like most professional players I always keep an eye on how the game is going in England and it's the kind of challenge that I know I will relish."
Thomas is one of a number of senior international players having signed on at Wasps for next season with England's James Haskell and Tom Palmer also joining Young's squad later this year.
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