Thompson joined the Vikings at the beginning of the season but failed to make an impact at the Stobart Stadium, and last month spent a period on loan with Harlequins.
He returned to Widnes last week but the two parties have now agreed a release.
Sporting Director Terry O’Connor said: “Sam asked for a release as he wasn’t enjoying his rugby at Widnes and we agreed to his request. On behlaf of everybody at the Club I would like to thank Sam for his efforts during 2009 and wish him well in his future career.”
Thompson is now rumoured to be on his way to Barrow, who Widnes defeated in Sunday’s Northern Rail Cup final.