The 33-year-old Hanbury has spent the last seven years with the Vikings but did not feature in any of their Super 8s Qualifiers fixtures as they failed to avoid the drop into the Championship and he has been given notice of termination.
Widnes chief executive James Rule said: "Rhys has created many special moments during his time at Widnes Vikings and can be proud of his contribution to the club.
"He leaves with the best wishes of everyone at the Vikings for the future."
All Super League contracts go back on the table following relegation, and clubs and players have 30 days to confirm their intention to renew or terminate.
Two other overseas players, Aaron Heremaia and Chris Houston, are retiring and Widnes are certain to lose the bulk of their best players, with the likes of Joe Mellor, Matt Whitley and Danny Walker all linked with moves to Super League clubs.
However, highly rated winger Owen Buckley has given them a boost by committing his future to the club.
The 19-year-old academy product, who has played in seven of the Vikings' last 11 matches, has signed a new contract that keeps him at the Halton Stadium until the end of 2020.
Buckley said: "I'm buzzing to have secured my future. It was a big commitment for me but it's one I'm ready to take and show to the fans how loyal I am to this club."
Widnes head of youth Phil Finney said: "Owen is a Widnes fan and knows the importance of the club to the local community and has been very clear about his aspirations to play for this club.
"I have no doubt that his passion and enthusiasm to represent Widnes will enable him to establish himself as a regular first-team player in the near future."
Buckley is the third player to announce his intention to stay with the Vikings since their relegation to the Championship was confirmed at the weekend, with overseas forwards Hep Cahill and Wellington Albert also signing new deals.
The Vikings, who have yet to make an announcement on the future of interim head coach Francis Cummins, conclude their 2018 campaign with a trip to Hull KR on Sunday.
