Germany's Stefan Luitz (2:38.33) finished third.
"The last two weeks have been really intense trying to find my speed and my confidence, because when you're injured you lose a lot of confidence," Hirscher said.
"It is really unbelievable but it's just one step further on the way back."
The win puts Hirscher one career World Cup win behind American Ted Ligety, his longtime rival who is still trying to find his form since coming back from back surgery in January.
Ligety (2:38.66), whose shaky second trip down the course landed him with a disappointing seventh-place finish, said his back felt fine during the race but he was undone by the course's slow, "grippy" snow on an unusually warm day.
With two months to go before the Winter Olympics in Pyeongchang, Hirscher will hope his giant slalom dominance will translate into Olympic gold.
Despite being the winner of a record six straight overall World Cup season titles, Hirscher has just one Olympic medal to his name, a silver in slalom from the Sochi Games in 2014.
(Reporting by Rory Carroll; editing by Clare Fallon)
