Switzerland's domination of the championships was expected to continue with reigning world and Olympic champion Marco Odermatt the pre-race favourite.
But Haaser jumped from fifth after the opening run raced in poor visibility to eclipse the Swiss and top the leaderboard with a rip-roaring second descent.
It was the biggest win of his career and delivered Austria's second gold medal of the championships on home snow and the first in the men's events.
Swiss Thomas Tumler finished 0.23 seconds behind in second place with teammate Loic Meillard in third place.
"I don't know what to say, it's such an unbelievable day," Haaser, who has never won a World Cup race but was second in the super-G earlier in the championships, said.
"I thought I was capable for top 10 today but not the top of the podium. Don't know where it came from."
When Haaser crossed the line for a combined time of 2:39.71, there were still four more racers to come including Odermatt and surprise first-run leader Timon Haugan of Norway.
Norway's Alexander Steen Olsen, fourth after the first run, failed to complete his second run and then Odermatt made too many mistakes as his chance of adding to his super-G gold disappeared and he finished off the podium in fourth place.
Haaser, and the huge crowd, held their breath as Meillard looked on course to go quicker but he leaked time through the closing gates.
It was left to Norway's Haugan to try and deny Haaser but he never looked like managing that as he dropped off the podium.
Action continues with the women's slalom on Saturday before the concluding men's slalom on Sunday.