The Swiss skier was eight fastest in the first run but far quicker than any of the other leading contenders in the second.
She finished with a combined time of 1 minute 55.03 seconds to edge compatriot Wendy Holdener by 0.16sec and Swede Sara Hector by 0.38.
Austrian Katharina Liensberger, who led after the first run, 1.50sec ahead of Rast, slid to fourth.
At 25, Rast took her second career World Cup victory.
The first came with another second-run surge in Killington, Vermont, in December.
In the absence of American Mikaela Shiffrin and Slovakian Petra Vlhova, both injured, Rast took the lead in the overall and slalom standings, just a few weeks before the world championships in Austria.
“It’s not bad,” Rast said.
“I read yesterday that Mikaela and Petra have won the last six slaloms here, so I’m very happy to have been able to add another name to the list of winners,” she added.
Rast leads Hector by 26 points in the overall standings and Holdener by 137 in the slalom.
Michelle Gisin, a double-Olympic gold medallist, was mobbed by Swiss team-mates after finishing 23rd.
The 31-year-old, whose only World Cup victory was in a slalom in 2021, had announced she was racing in the discipline for the last time.
© 2025 AFP