The American had a commanding lead after her first run down the icy piste and then played it safe on the second to beat Italian Federica Brignone (33) by 0.38 seconds.
Sweden's Olympic giant slalom champion Sara Hector (31) was third after finishing the first run in second place.
The victory, in the final giant of 2023, was Shiffrin's first in five giant slaloms so far this season and 22nd of her career in the technical discipline.
"My big, big goal right now is to close the gap on the GS standings so I was like 'now I want to win it but I really don't want to mess it up'," she said.
"I really wanted to push on the second run, a couple of small errors, nothing big, but I am happy with the skiing anyway."
Shiffrin won seven giant slaloms last season on her way to the World Cup title but her first of that campaign also came on the same day a year ago.
"When you miss a win for so long in the season it's easy to be like 'I am doing things wrong'," she said. "But tactically the season is a marathon not a sprint..."
Shiffrin leads Brignone in the overall standings by 163 points but is third in the giant slalom rankings, 80 behind the Italian and 45 behind Switzerland's Lara Gut-Behrami (32).
A slalom is scheduled in Lienz on Friday, with Shiffrin a favourite to extend her tally to 93 wins.
