Thomas left his Worcester head coach role in January 2022 and has previous coaching experience with Bristol.
The 39-year-old is a double Grand Slam winner with Wales and played for Swansea, Ospreys and Worcester during his 14-year career.
“I’m pleased to be here and everyone has been so welcoming,” said Thomas.
“From speaking to Wardy (director of rugby Ben Ward), it just felt like a natural fit.
“I’ve played and coached in this league before, so I know how tough the Championship is and hopefully I can make a positive difference.”
