Weirather turned in a time of 1 minute, 17.38 seconds on the course at St. Moritz and edged Kajsa Kling of Sweden, who finished a surprising second, by 0.31 seconds.
Weirather, who took two second places last weekend in Canada, said afterward the smoother skiing style she developed after recovering from injuries was deceiving. "I didn't really feel I was going that fast today," she said. "I thought I was turning too round and that nothing was happening."
Kling continued an impressive run that included top-10 finishes in the two Canadian events last weekend. "The slope is nice and the course set was very rhythmical," she said. "That really fits me well."
Anna Fenninger of Austria came in third 0.39 seconds behind Weirather. Austrians Elisabeth Goergl and Nicole Hosp took fourth and fifth places.
Veteran Stacy Cook was the top American finisher in 16th place. Lindsey Vonn did not compete Saturday as she continued to gingerly get back in form for the Winter OIympics.