A strong ball-carrying centre with excellent defensive skills, Thomas has also previously represented Cardiff Blues and captained Wales at all age levels up to Under-21.
London Wasps Director of Rugby Dai Young said: "Lee is a hard-hitting centre who will bring something different to our mid-field. He has plenty of experience in this league and will be a very good addition to our squad especially considering he can also cover fly-half and kicking duties very effectively."
Thomas said: "I am delighted to be returning to England to be part of an exciting squad and coaching team. I have enjoyed my time in France but feel it is the right time to return to England and be closer to home. I know Dai Young well from my time at the Blues, and am looking forward to working with him again."
Thomas is the second signing in a week for London Wasps for next season with hooker Rhys Thomas also signing from the Cardiff Blues. England internationals James Haskell and Tom Palmer have also signed for Wasps next season while this week the club announced contract extensions for Hugo Southwell and Tim Payne.