Shiffrin led by 0.60 seconds after the first leg in Finland and comfortably held on to her advantage second time down the course in dark, gloomy conditions to beat Austria's Katharina Liensberger by 0.79sec.
Lena Duerr of Germany took third place at 0.83sec. Duerr finished a distant second behind Shiffrin in last season's slalom standings.
"Amazing way to start the slalom season. I'm super happy," said Shiffrin, who endured an injury-hit 2024 and surrendered her overall crown to Switzerland's Lara Gut-Behrami.
Shiffrin, a five-time winner of the women's globe, triumphed for the eighth time in Levi, a small resort located some 170km north of the Arctic Circle.
The winner in Lapland traditionally receives a reindeer, which they get to name but stays in Finland.
"You never forget (about the reindeer)," she said. "I wasn't thinking about it on the run."
It was the 61st slalom victory of her career and came with Shiffrin's main rival, Petra Vlhova, still absent as she continues her recovery from a serious knee suffered 10 months ago.
Shiffrin and Vlhova have combined to win every race held in Levi since 2014, when Tina Maze won in her final season.
Vlhova has admitted she does not know when she will be back on the pistes after knee surgery and has written off the start of the winter.
In increasingly difficult conditions, with heavy snowfalls as the morning progressed, the first run in Levi was marked by around 20 run-offs.
That didn't disturb Shiffrin though as the 29-year-old took charge, ahead of Duerr, with Zrinka Ljutic of Croatia third at 0.73sec.
Ljutic's podium bid crumbled during her second attempt as Liensberger, fourth after the opening run, muscled her way onto the top step before being dislodged at the end by Shiffrin.
"(It was) a little bumpy. I was getting twisted sometimes but then kept fighting," said Shiffrin.
"It wasn't the perfect tempo but enough for really good turns that it worked really well in the end. Really solid run in conditions that were not too easy."
Shiffrin moves top of the standings after bouncing back convincingly from a fifth-place finish in last month's season-opening giant slalom on the Soelden glacier.
The women head to Gurgl in Austria next Saturday for another slalom before three consecutive weekends in North America -- two in the United States and one in Canada.
