Widdop will join the Wolves in 2020, with the Dragons releasing the 29-year-old only on the grounds he returns to St George if he does come back to the NRL.
“I’ve made a big decision in my career having decided to move back to England next year,” Widdop told the club’s website.
“It certainly hasn’t been the easiest decision but sometimes in life we have aspirations and playing in the Super League has been one of mine for a while now.
“The Dragons have played a big part in my career. I’ve been here a long time now and to get to where we have with the roster we’ve got, to leave it all behind especially being the captain will be difficult.
“This situation has been handled in the most professional way possible. I worked really closely with the club who have been great. We worked towards getting the best outcome and I thank the Dragons for that.
“I still have the 2019 season to come. The reason we play this game is to win premierships. We have the squad to win it this year and I’ll be doing everything possible to achieve that.”