The Austrian, seeking an unprecedented fifth consecutive overall title, became the first man to win three successive giant slalom races at the Alta Badia venue, in Italy's Dolomite mountains.
He claimed the World Cup lead from Aksel Lund Svindal, moving 20 points ahead of the Norwegian -- who was 28th, following his two speed-event wins in preceding days at nearby Val Gardena.
Hirscher claimed his fourth victory this season, and his third in giant slalom -- a technical discipline where he already leads by 134 points after only four rounds.
He was 0.19 seconds ahead of Norway's Henrik Kristoffersen after the two runs, while France's Victor Muffat-Jeandet was third.
"I skied at my 100% limit in the second run," Hirscher said after the 35th win of his career put him one behind sixth-placed compatriot Benjamin Raich in the all-time standings.
"It was difficult snow, not as icy as we're used to here in Alta Badia and I think that Henrik skied a smoother run, but I gave it all my power and in the end I am really happy to come away with the win."
In Sunday's women's action, Eva-Maria Brem made it a double giant slalom success for Austria by winning in Courchevel, France.
Switzerland's Lara Gut, who won twice at Val d'Isere in Friday's alpine combined and Saturday's downhill, tied for second with Norway's Nina Loeseth.
Gut extended her overall World Cup lead to 58 points over American star Lindsey Vonn, who was 13th following her crash on Saturday.
"I didn't really feel very balanced today," Vonn said. "I think I just rung my bell a little bit yesterday."
Brem claimed her second career World Cup victory, having been runner-up in the previous giant slalom races at Aspen and Åre.
"Every season I just need that one moment when everything gets together -- that one run, like the second one in Aspen when I felt, 'This is how I need to keep this working,'" the 27-year-old said.
"I knew what I had to do, but waiting at the start was tough. That second victory was beautiful too and that made me cry."