The 24-year-old spent last season on loan at MK Dons and scored 22 goals in all competitions as they claimed a place in the Sky Bet Championship.
Latics boss Gary Caldwell is hoping he can repeat that fine form for Wigan.
“Will is the one we were after, he’s a goalscorer, that’s what he does and why we’ve signed him,” he told the club’s official website. “There were other clubs in the chase, including from the Championship, but he’s chosen to come to us and we are very happy about it.
“He’s a proven goalscorer at League One level and had a superb season last year on loan with MK Dons, so he knows this division and is a real quality addition.
“The squad is beginning to take shape now and there is a great atmosphere in the camp, the addition of Will and Craig Davies last week has added to that because these are players with a track record for scoring.”
Grigg becomes Wigan’s eighth summer signing, joining David Perkins, Richard O’Donnell, Craig Morgan, Donervon Daniels, Max Power, Sanmi Odelusi and Craig Davies.