The 22-year-old had a loan spell at Fir Park before arriving at Deepdale from Manchester United in January 2016 as has signed a one-year deal with the Scottish Premiership club.
Having overcome a knee injury Grimshaw made his PNE debut as a right wing back on the opening day of last season at Reading.
He only made seven appearances in 2016-17 however, rarely appearing in his preferred midfield position, before moving to Chesterfield where he finished the campaign on loan.
Grimshaw’s only involvement under Alex Neil this season has been playing the first 65 minutes of the League Cup exit at Accrington.
His contract would have been up at the end of the season but was cancelled early to allow him to make the deadline-day move.