Former Pompey forward signs for Wigan
The Scot will pen a three-year deal on July 1 at the DW Stadium following the expiry of his Pompey contract.
Naismith departed Fratton Park earlier this month after contract negotiations failed to reach a conclusion.
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Hide AdThe ex-Rangers forward made 94 appearances and scored 21 goals at Fratton Park after joining from Accrington in 2015.
Yet Naismith struggled to show his best form under Kenny Jackett last season, registered just three times in 31 matches.
However, the 26-year-old will reunite with the boss he admitted gets the best out of him at Wigan.
Naismith was Cook's talisman on the road to Pompey winning the League Two title in 2017, scoring seven goals and registering as many assists in the final 11 matches.