Trailing 17-5 at half-time, Chris Boyd's men looked to be in all sorts of trouble.
But the green, black and gold produced a remarkable second half showing to score four tries and win it 31-7 to seal a memorable success.
It was Saints' first win in France in any competition since 2011, and the bonus point victory means Saracens and Gloucester now have to secure five-point wins in their final group games on Sunday to deny Boys's men a place in the last eight..
Saracens are at home to Racing 92 (ko 1pm) and Gloucester travel to Toulouse (ko 3.15pm). Both matches are live on BT Sport.
It was Saints' first win in France in any competition since 2011, and the bonus point victory means Saracens and Gloucester now have to secure five-point wins in their final group games on Sunday to deny Boys's men a place in the last eight..
