The 28-year-old midfielder left Sixfields in the summer, despite the offer of a new deal, to sign a two-year contract at Plainmoor.
"He is a big player and somebody I've always liked," said Torquay boss Alan Knill. "He is used to being in winning teams and knows what it takes."
His new side, like the Cobblers, have made a slow start to the 2013/14 campaign with two points from their three games to date. Harding has started all three.
Elsewhere in League Two, young Sunderland midfielder Billy Knott has joined Wycombe Wanderers on loan while Bury have continued their squad building with the capture of the experienced Tommy Miller.
***Update***Aidy Boothroyd was pleased with the way his side won a friendly against Watford on Tuesday afternoon, Clive Platt and Chris Hackett getting valuable game time and both JJ Hooper and Ishmel Demontagnac scoring goals.