The Austrian saw off competition from Pep Guardiola, Jose Mourinho, Ole Gunnar Solskjaer, and David Moyes to win the award for the first time.
As previously reported, Hasenhuttl was nominated following a six-game unbeaten run in the concluding month of the season.
In that six-game unbeaten run, the South Coast side picked up three impressive wins over Manchester City, Bournemouth and Sheffield United.
Saints beat Manchester City 1-0 at home towards the beginning of the month in an impressive defensive display before a 2-0 victory away to Bournemouth and finishing the month off with a 3-1 win over Sheffield United on home turf.
In between the Manchester City and Bournemouth victories Saints recorded draws over Manchester United, Brighton, and Everton giving them a total of 12 points from a possible 18 in July.
It is the first time that Hasenhuttl has picked up the accolade having previously been nominated in January but missing out to Liverpool's Jurgen Klopp.
Hasenhuttl took over the helm at Southampton in December 2018 having managed in Germany since 2007 and coached there since 2005.
Since arriving in England the 52-year-old managed to save Saints from relegation last season and has steered them to an impressive eleventh placed finish this campaign.
In part thanks to him getting the best out of striker Danny Ings, even turning the forward into a threat for the golden boot this season, and gaining some impressive defensive performances out of his side.
No doubt the Austrian will look to improve on their position as 'best of the rest' this season and challenge for a Europa League spot - a tournament Southampton haven't qualified for since 2016 where they were knocked out in the group stage.