The 32-year-old centre joins Trinity from Wigan Warriors, having played for his hometown club in their 2022 Cup triumph against Huddersfield Giants.
That was his second spell with Wigan and his other clubs include Hull KR, Catalans Dragons and Leigh Centurions, after he began his professional career in rugby union with Sale Sharks and then Leeds-based Yorkshire Carnegie.
“I am delighted Iain Thornley has agreed to join us next season,” Powell, who was appointed this week, said. “He is a big, powerful centre who has been outstanding for Wigan whenever he has played over the last few seasons. He will add leadership and experience to what is going to be an exciting backline.”