The 26-year-old has spent the last four seasons at Widnes, racking up 87 appearances.
His 2015 campaign was curtailed by a knee injury, but he will be ready to take a full part in preseason.
“I am absolutely delighted to have joined Wigan,” Isa told the Warriors’ official website.
“There were a few offers on the table but I simply couldn’t turn down this chance when it came along.
“I’m really grateful to Shaun Wane and Kris Radlinki for this opportunity, I’m working hard on some rehabilitation stuff at the moment but will be fit and raring to go next season.”
Wigan coach Shaun Wane is looking forward to working with a player of such ability.
“I’ve been monitoring Willie for some time and didn’t think twice about completing this deal when we had the chance,” he said.
“He is highly thought of here at Wigan, our senior players who we consulted all commented that he is a fierce competitor who is tough to play against.
“At 26 years of age his best years are yet to come. He is a big pacey player who can play in a number of positions. I think he will really suit our style and the Wigan fans will really warm to him.”
Wigan director of rugby Kris Radlinski confirmed that the Warriors had beaten competition from other clubs to attain the player’s signature.
“Having met with Willie we were really impressed with his desire to join the Wigan club,” he said.
“He is an established Super League player and we’ve beaten off stiff competition for his signature. Willie was actually offered more money elsewhere but he desperately wanted to come here and we are delighted about that.
“On the field he is a fierce competitor who certainly knows how to handle himself in the Super League competition. Off it he is a genuine guy who is willing to learn and improve himself and his game.”