Earl worked his way through the North End Academy and featured 67 times in the Sky Bet Championship for the first team, 29 of those games coming in the 2021/22 season.
The 23-year-old also has experience at Sky Bet League One level, having had loan spells with Bolton Wanderers, Ipswich Town and Burton Albion.
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Town’s head coach, Scott Brown, said: “Josh is a player that we’ve been looking at since the day we first arrived at the football club and we’ve finally managed to get the deal over the line, which is great to see.
“The chairman (Andy Pilley) and the backroom staff are delighted to get the deal done as he is an individual with great experience in the Championship.
“When he plays in League One with us, we’re wanting him to lead by example and show the qualities that we all know he’s got.
“He’s a left-footed centre-back who wants to push forwards and likes to be aggressive in the air – and we’re all ready to work together to push on ahead of the new season.”
Town have also confirmed they will play Tranmere Rovers in a behind-closed-doors pre-season friendly at Poolfoot Farm.
It takes place this Saturday, July 2, before the Cod Army squad heads to Croatia for a training camp the following week.