The Norwegian finished with a score of 25.94, while America's Shannon Bahrke claimed silver with 25.06 and Japan's Tae Satoya the bronze on 24.85.
Traa, who won bronze at the 1998 Nagano Games, qualified for the final 16 in first place, while Bahrke started in fifth and Satoya in sixth.
An impressive performance by Bahrke saw her in gold medal spot with just Traa left to run.
But with the pressure on Traa produced a superb triple twist, to win the judges points, and a fast time to take the title away from the American.
Another American, Hannah Hardaway, who currently lies second behind Traa in the World Cup standings, qualified in third but finished a disappointing fifth.
"I was so nervous while I was waiting for my score because my second air (jump) wasn't so good," Traa said.
"I'm just so happy that I made it."
Bahrke thanked the 10,000-strong crowd fans after her silver medal performance.
She said: "There are so many people here... all my family and my friends and my boyfriend and his parents and all these wonderful Americans.
"I just used the crowd to kick me up another notch."
British mogul skiers Joanne Bromfield and Laura Donaldson failed to qualify for the final after finishing 28th and 29th respectively.