The Cobblers announced their swoop for Southampton academy product Turnbull, who had been on loan at Ladbrokes Premiership side Partick Thistle from the Sky Blues.
Turnbull, a former England Under-20 international, previously spent two season-long loan spells with Swindon. He moved to Partick last summer.
"Jordan is a left-footed centre-back, and he will be a good addition to the squad," Northampton manager Jimmy Floyd Hasselbaink told the club's official website.
"Despite only being 23, he already has an excellent amount of experience under his belt, having played more than 150 senior games, and there aren't many players of that age with that level of experience."
Turnbull's move is subject to international clearance, as he has recently been playing in Scotland.