The 26-year-old Ireland international heads to the Hawthorns for an undisclosed fee following two seasons in the north-west.
Baggies boss Tony Pulis is delighted to bring McClean back to the top flight and he told the West Brom website: “He’s got Premier League experience from his time with Sunderland.
“James has had a couple of indifferent years with Wigan but he did well at Sunderland and we’re very pleased to have him on board.
“I am absolutely confident that he can fulfil the potential to come back and be a big player for us.
“He’s got pace, he’s left-footed, so he gives us a good balance in the side – and he can score goals.”
The player himself, added: “I’m delighted. I’ve played in the Premier League before and I’m with a stable top flight club and one that wants to push on.
“Tony Pulis told me what his plans were, where I would fit in. It was all very positive. The Premier League is where any player wants to be.
“Everything about this move seems right for me. Once the Albion came in this was a move I was keen to get done. This is a big club and a new chapter for me.”
McClean made 70 appearances for Sunderland before signing for Wigan in the summer of 2013.
He collected 86 appearances for Wigan and scored 10 goals, and has been capped 30 times by Ireland.