The 20-year-old had already claimed the combined event and took silver in the super giant slalom.
And on Friday, she will have a chance in the giant slalom to become the first person with four Alpine medals in one Olympics.
As heavy snow caused difficult, slippery conditions, she triumphed in the slalom over France's Laure Pequegnot and Swede Anja Paerson.
Kostelic clocked a combined time of one minute, 46.10 seconds for the two runs.
"It was kind of easy going after my first medal - my job was done"
Double gold medallist Jan Kostelic
Pequegnot won the silver in 1:46.17, with Paerson finishing in 1:47.09.
Kristina Koznick, America's last realistic chance for an Alpine medal, crashed on the first run.
Among those who did not finish the first run was Christel Pascal-Saioni of France, a pre-race medal contender.
Pascal-Saioni was left angrily pounding her right fist into the snow after hitting a gate near the top of the course and losing her left ski.
Britain's Chemmy Alcott did not finish, although team-mate Emma Carrick-Anderson did manage to come 19th.
Meanwhile, Kostelic was savouring another success.
"t was kind of easy going after my first medal, my job was done. I'm just enjoying myself and skiing."
She admitted the conditions had an effect on the competition.
"I skied the best way through a bad course. It was breaking up, it was rutty," she said.
"I had a clean run, I didn't really have mistakes."