The win was a second of the season in the LV= Insurance County Championship Division One for Northants, as they got home with 20 balls to spare after a thrilling final day.
The County had been frustrated for the first two sessions of the day as Gloucestershire skipper Graeme van Burren scored a brilliant 127 not out to steer his side from being in serious trouble at 135 for five the start of play, to 363 for nine - a lead of 201.He then declared at tea to set up a run-chase as bottom-of-the-table Gloucester chased a first victory of the season, with Northants needing 202 from a minimum of 37 overs.
Ryan Rickelton, in his final game before joining up with the South Africa Test squad, hammered 68 from 47 balls and opener Will Young 43 from 39 as the County got off to a flyer at 115 for one in the 15th over.
Wickets then fell steadily as the pressure increased, but a crucial 46-run partnership between Lewis McManus (30) and James Sales (17) got Northants to the brink of victory.
There was a late clatter of wickets as Zafar Gohar finished with five for 62, but Simon Kerrigan kept his nerve to hit the winning boundary in the 34th over.
"What an incredible game of cricket," said Sadler.
"All credit to Gloucestershire, who could have settled for a draw, but set up a great finish. I thought van Buuren's century on a turning pitch was a fantastic effort.
"We were always intent on going for the runs.
"Will Young and Ryan Rickelton showed they are world class and how cool were Lewis McManus and young James Sales at the end?
"At times we looked in total control and at others we were under pressure, but we hung in there and got the result at the end of a fantastic day."
Van Buuren had no regrets about setting up the dramatic finale, and praised both sets of players and umpires for ignoring bad light in the closing stages to ensure the match was completed.
"I am so proud of my team," said van Buuren.
"We have put in a tremendous amount of effort over four days in a game that was a fantastic advert for cricket and the County Championship first division.
"It has been a lean season for me, so I was delighted to get some runs for the team.
"I don't think there is anything I would have done differently. One or two key moments didn't go our way.
"The light was an issue at the end, but both teams wanted to stay out there and all credit to the players and umpires for completing a wonderful match."
Northants now take a break from red ball cricket for the whole of August, and turn their attentions to the Royal London One Day Cup.
They go into that break sitting in fifth place in the County Championship table, 31 points above the bottom two and 25 points off third, with a game in hand.