Widdop, who emigrated with his family at the age of 16, won Premiership honours with Melbourne after working his way up through the club’s junior system and has spent the last five seasons with the Dragons.
When he announced his desire to finish his career in England, it sparked a flurry of interest and the big-spending Wolves won the race for his signature.
Warrington chief executive Karl Fitzpatrick said: “This is arguably the biggest signing in the club’s history and quite possibly in the history of Super League.
“Bringing in this world-class player is a major boost for the sport in this country.
“In 2020 Gareth will be a fantastic addition to an already-formidable squad. This, once again, demonstrates the ambition of Warrington Wolves.”
Widdop, who has been an England regular since 2010, will be reunited with Warrington head coach Steve Price, who also signed him for the Dragons in 2014.
“He’s a terrific person who sets high standards for himself and is a really good family man,” Price said.
“He’s matured into an outstanding leader and has played on the biggest stage in the game at international level.
“Gareth’s a player with integrity and a high level of experience; he’ll complement our team in 2020 and help take us to another level.