The midfielder joined the League One side on loan in September and after extending Chaplow's loan, the Saints have decided to make the move a permanent one.
Chaplow joined the Lilywhites in January 2008 from West Brom following a loan spell at his new club.
"I knew what I was coming into as I've been here on loan before," Chaplow told the club's official website. "I know it is a great club, I knew the gaffer had done a great job at Scunthorpe and is doing a great job here, then I spoke to a few people like Neal Trotman who was here last year and he said that the lads were great, so I knew that I was coming into a good, ambitious club that is going places and I'm thoroughly enjoying it really.
"Coming down here was about getting some games and I hoped that if the gaffer liked what he saw in me a permanent deal could be put in place, which is the way it has panned out so I'm ecstatic.
"I've enjoyed getting on the pitch again which is where I want to be and contributing where I can to help the team win, so fingers crossed we can push on now and get where we need to be next season."