Wakefield chairman Michael Carter says Kyle Amor was sold now rather than the club losing him for nothing at the end of next season.
Amor, 26, has penned a four-year with St Helens, with the Wildcats receiving a fee of £50,000 for the Cumbrian.
He is the second player, following Tim Smith, to be sold by the struggling club in recent weeks.
Carter told the club’s official website: “Kyle came to me and let me know that he would like to leave the club and that he wouldn’t sign a new contract when it was due to expire at the end of next season.
“We were left with the option of seeing him leave at the end of next season for nothing, or securing a transfer fee for a player that was much sought after. On that basis, and with Kyle’s desire to leave, it was a straightforward decision.
“Kyle has been fantastic for us in the last few seasons, and along with the coaching staff has got his game to a level where top clubs wanted him in their squad. We wish Kyle all the best and thank him for his efforts with the club.”
Amor made 73 appearances for Wakefield, having first joined on loan from Leeds in 2011.