The Fir Park club confirmed in a statement this morning that the forward - who had indicated a desire to leave the club at the end of his current contract - would join the Lilywhites on 1 January.
Moult, who has netted 50 goals in 98 appearances for the Steelmen, is currently injured but could feature for Motherwell if he returns to fitness before the start of next month.
The former Wrexham striker was offered several new deals in a bid to keep him in Lanarkshire but chose to return south of the Border.
Manager Stephen Robinson hailed Moult’s ‘terrific contribution’, adding: “We are sorry to see him go at this stage but he leaves with our thanks and best wishes.
“The player made clear he would not be extending his stay after the end of the season.
“So when the offer came from Preston, we had to weigh up whether the money would benefit the club against the risk and reward of keeping him and then allowing him to leave on a free.
“We accepted the offer based on that it would be best for all parties now that a club has agreed a contract with the player.”
‘Well chairman Jim McMahon added: “The board would like to sincerely thank Louis for his efforts and contribution during what has been a sterling two-and-a-half years at Fir Park.
“We were obviously desperate for the player to stay with us and made him numerous offers over the last 12 months, the last of which being literally everything this club can realistically afford, given our size.
“However, we also understand and appreciate the player was made a life-changing offer from Preston and we completely respect his wish and desire to move on now.”
McMahon confirmed that the club had been working on a replacement for Moult ‘for some time now’, adding: “When the window opens, we will look to strengthen our squad in a number of different areas.”