A Grand Slam winner with Wales Under-20s in 2016, Thomas made 18 appearances for the Ospreys before moving to play club rugby in New Zealand this year.
Thomas began his career with Aberavon and Swansea before earning PRO14 experience with the Ospreys and also had a loan spell with the Dragons.
Tigers head coach Geordan Murphy said: “Joe has a very good skill-set, he was highly rated on his way through the ranks in Wales and now he is keen to come in here and take on a new challenge with Leicester.”
The 23-year-old Thomas, who returns to the UK after a spell with Waikato club Otorohanga in New Zealand, said: “I’m really excited about the opportunity to join Leicester Tigers. They are a massive club in the Gallagher Premiership with a big history of success.
“I can’t wait to get started. It will be great to be able to learn off some great coaches and compete and play alongside great players.”
Tigers have previously announced the signings of Jordan Taufua, Tomas Lavanini, Calum Green, Noel Reid, Jaco Taute, Nephi Leitigaga and Charlie Clare for the new season.