Harrop spent the second-half of last season on loan in League One with Ipswich Town.
He returned to PNE for pre-season but didn't make the 25-man squad registered. Injury restricted his training and prevented him playing in the reserves during the first-half of the season.
The 26-year-old will now hope to get a regular run of football at Fleetwood who are battling to pull away from the League One relegation zone - they are in 19th place and two points clear of the bottom four.
There will be a familiar face to Harrop at Highbury, with former PNE skipper Tom Clarke now with Fleetwood.
After wrapping-up the move to the Lancashire coast, Harrop said: "The most important thing is playing football and enjoying myself, doing what I love.
"I don't want to be on the bench or sat in the stands anymore, I want to be on the pitch enjoying my football.
"It's like any footballer, you can't play one game here and one game there and be amazing. You need back to back games, it takes five or six games to get match fit, it takes five or six games to get into the rhythm of things.
"That's what you need as a player, you need regular games to get to your top level of fitness or awareness on the ball and to be able to broadcast what you can do. "
Commenting on the injury he had earlier in the season, Harrop said: "It was a tendon injury and inflammation that was flaring up every time I trained.
"So it was just a matter of getting that right, I was always playing with a little bit of something.
"It came to a stage where I needed to get it right so I could focus on my football. It took a good four months to get it right but now I don't have to worry about that injury now and I can focus on my football.
"I've been in the gym working hard and training for the last month and a bit now so I'm ready to go. "
Harrop is is the sixth player to leave North End on loan during the January transfer window.
Jordan Storey (Sheffield Wednesday), Adam O'Reilly (St Patrick's Athletic), Jamie Thomas (FC Halifax), Jacob Holland-Wilkinson (Bamber Brige) and Lewis Coulton (Warrington Town) have gone out in the past week.