Ledecka, favourite in the snowboard parallel slalom in a week's time, clocked 1min 21.11sec down the polished 2km-long Jeongseon course to edge defending champion Anna Veith of Austria by one-hundredth of a second.
Liechtenstein's Tina Weirather took bronze.
Not only did Ledecka deprive Veith of what looked like a rare double, she pushed Vonn back into sixth spot, the well-known American paying the price for a massive error that saw her lose valuable time at the bottom of the course.
"It is definitely shocking," Vonn said of Ledecka's win.
"She beat me in training in Lake Louise and that was also surprising. It is not that she did not deserve it but there is a lot of pressure on the favourites."
Ledecka was also surprised.
"All of the other girls did not risk a lot," she commented. "There must be a lot of pressure on them. I was just trying to do my best run.
"I am so surprised about all of it. I am really trying to win and do a good run every time but I did not really realise that this can happen."