The victory, in her last race before the Milano Cortina Winter Olympics, was the record-extending 108th of her World Cup career and 71st in slalom.
Shiffrin is the first skier, male or female, to win nine World Cup titles in a single discipline. American Lindsey Vonn won eight in downhill, and Austrian Marcel Hirscher won eight overall globes.
Shiffrin was 1.26 seconds quicker than Switzerland's Wendy Holdener after the first run and was also quickest in the second leg to win by a huge margin of 1.67 seconds.
Switzerland's Camille Rast was second and Germany's Emma Aicher third, with Holdener dropping to fifth.
Shiffrin, the clear gold-medal favourite in the discipline, has won seven of the eight slaloms this season and finished second in the other.
She now has an unassailable tally of 780 points with world champion Rast 288 behind and two slaloms remaining after the Games.
"Every race is a little bit different, the first run for me was maybe an advantage to go bib one, because I didn’t know how the surface was going to feel," said Shiffrin, whose first World Cup race was in Spindleruv Mlyn in 2011.
"I didn’t have expectations and I kind of just skied as hard as I could. The combination on the second run today -- I was a little bit concerned I was going to mess it up. It’s wonderful to be consistent and fast, but every time I ski I feel like I could be flying off the course at any moment."
The podium was Shiffrin's second in two days after she finished third in giant slalom, her first top-three finish in that discipline in two years, also further raising Olympic hopes.
Rast, who said she had felt unwell last week and had not started the race in top condition, looked forward to some time at home before travelling to Italy.
The women's Olympic slalom in Cortina d'Ampezzo is scheduled for February 18th.
Shiffrin went to the last Olympics in Beijing in 2022 as a medal contender in six events but crashed out of three and failed to get on the podium in the others.
The American won slalom gold in 2014 and was the giant slalom champion in 2018.