Wakefield Trinity sign Wigan Warriors outside back to kick off recruitment drive under new regime
The 32-year-old scored five tries in 13 appearances in an injury-hit second spell at boyhood club Wigan, including a start at centre in last year's Challenge Cup final.
"I am delighted that Iain Thornley has agreed to join us next season," said Powell.
"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 be going to be an exciting backline."