Shiffrin, 18, plowed through a mix of falling snow and rain to reach the top of the podium and maintained her lead in the women's standings in the event.
"It was tough conditions but I was ready for it," said Shiffrin, who turned in a two-run time of 2 minutes, 00.41 seconds, edging Sweden's Maria Pietilae-Holmner, who finished with a 2:00.54.
Shiffrin noted the sloppy conditions required race officials to set up a course for the second run that was little changed from the first run. "I felt like I didn't need to worry that much ... . This allowed me to relax and just go for it," she said.
Nastasia Noens of France placed third with a 2:01.03. Maria Hoefl-Riesch of Germany, the leader in the overall women's standings, did not complete her second run.
Swim Week Miami Beach 2014 [PHOTOS]