The 24-year-old, who joined Sunderland for a fee of around £250,000 from Derry City in 2011, is expected to join the Latics in a £2 million move.
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"I'm delighted it's all been completed and I can say I'm a Wigan player," he told the club's official website.
"This is a fantastic club and after speaking with the Manager I can't wait to meet the rest of the team, start training and hopefully make my debut very soon with the aim of helping us get back to the Premier League."
Despite impressing in his debut season with the Black Cats, McClean suffered a downturn in form during his second season in England.
He made a total of 36 league appearances for Sunderland last season, scoring two goals, and saw little action in Sunderland's recent pre-season friendlies, making just one substitute appearance in the Barclays Asia Trophy against Manchester City.
Meanwhile, Derry, McClean's former club, are reportedly set to benefit from the deal by collecting a fee of £300,000 due to a 15 per cent sell-on clause.