Signed from Burton Albion last season the frontman was never in the plans of new boss Gary Johnson, who confirmed at the weekend that Harrad was set to leave.
Having lost their top striker, Ryan Lowe to Sheffield Wednesday, League One Bury moved quickly on deadline day to add Harrard to their ranks.
Johnson told the official club website that as well as Harrad falling down the pecking order over the summer, offloading one of Northampton's 'top wage earners' would help to balance the books at Sixfields.
Rochdale had also been linked with a move for Harrad while former club Burton also showed an interest.