"It did not feel that good," said Shiffrin. "I didn't expect to come down with a green light. It's been one of those days."
The American, who won last year's season-ender and the first four races in this campaign, finished the first run slowly and was fourth fastest.
On deteriorating snow, she was fastest in the second run to edge Swiss skier Camille Rast by 0.09sec with Albania's Lara Colturi 0.57sec back in third.
"Today was hard and you can see the skiing from these women," said Shiffrin.
"It was a really hard day today. Tough conditions and it was a really big fight. And the pressure's on and I did my best, best possible run."
Shiffrin moved to a maximum 500 points from five slalom races, already 220 points clear of second-place Colturi. In the overall standings, Shiffrin has 698 points, 195 ahead of Rast.
When asked if her final race of 2025 marked a perfect end to the year, Shiffrin laughed.
"'Perfect'? I don't know if that's the word, but it certainly was an end," she said.
