George Pisi, Jamie Elliott, Samu Manoa and George North backed up a try brace for lock Christian Day as Saints denied Bath their 12th consecutive win in all competitions.
The league's most accurate goalkicker Stephen Myler added 13 points to ensure Bath left Franklin's Gardens without even a losing bonus point.
Bath raced into a 13-0 lead thanks to Kyle Eastmond's try and two penalties and a conversion for George Ford.
But the Blue, Black and Whites' best record in a decade came crashing down as Northampton scored twice to turn the game when Bath prop Paul James was in the sin-bin.
Day's second score opened the second half in style for Saints before Bath rallied with a try for Rob Webber.
Manoa and Leroy Houston traded tries as Bath battled for a final-quarter turnaround, before North added the killer blow on the stroke of full-time.
Saracens wrapped up an easy 26-8 Premiership victory against Worcester to remain in charge at the top of the table.
The Londoners were favourites to win by a bigger margin but their four tries, which came via wing David Strettle, hooker Schalk Brits, full-back Alex Goode and fly-half Charlie Hodgson were good enough.
The stand-off converted three of the touchdowns,
Worcester tried to add to a sole penalty from outside-half Ignacio Mieres and try by number eight Leonardo Senatore during a scoreless second period, but it was all in vain.
Saracens had not won in the league at Sixways Stadium since a 21-16 victory in October 2007, but that run of defeats was looking odds-on to end by half-time as the Londoners stretched out to a four-try bonus point lead within half-an-hour.
Leicester held off a tremendous fightback by Sale to beat the Sharks 30-23 at Welford Road.
It was a much needed boost for the Premiership champions who started the day in seventh place after last week's thrashing at leaders Saracens.
They owed the win to a great first half performance which earned them two tries and a 20-6 half time lead.
Star of the show was young Welsh fly-half Owen Williams who had a fabulous first 40 minutes and finished with 15 points from a faultless kicking display.
But Sale hit back after the interval with two tries of their own and a late penalty by replacement Joe Ford earned them a bonus point.
Harlequins protected fourth place in the Aviva Premiership but were unable to produce the bonus point that looked certain following a dominant first half in the 22-6 win over Exeter Chiefs.
A crowd of 74,827 at Twickenham saw Nick Evans, Charlie Walker and Mike Brown score tries as the 2012 champions entered the interval 19-3 ahead.
But they could only then manage a penalty from Evans as the entertainment produced in the first half of 'Big Game Six' was replaced by an arm wrestle that finished 3-3 on the scoreboard.
Former All Blacks fly-half Evans endured a mixed afternoon, kicking two conversions and a penalty but missing eight points from the kicking tee.
Hooker Dave Ward finished man of the match after proving a constant thorn in Exeter's side at the breakdown and there were also notable performances from centre Matt Hopper and wing Walker.