Kostelic finished in a combined time of two minutes 30.01 seconds to claim her third gold and a record fourth alpine medal at Salt Lake City.
Anja Paerson of Sweden was over a second behind with a time of 2:31.33 to take silver, while Switzerland's Sonja Nef grabbed bronze in 2:31.67.
But it was Kostelic's day, having become the Winter Olympics most successful alpine skier, but the Croatian was just relieved that it was all over.
"I didn't care about the record - I just take one race at a
"I've got no energy left," she said, "I think I'll just go and pick up my medal and go to bed."
Kostelic joins Jean-Claude Killy and Toni Sailer as the only alpine skiers with three golds in the same Olympics.
The 20-year-old Croatian had already won gold medals in the slalom and the combined event, as well as a silver in the Super-G.
Killy won three gold in 1968, while Sailer did it in 1956, but Kostelic was more concerned with winning than breaking records.
"I didn't care about the record - I just take one race at a time," she said.
Despite an impressive medal tally going into the event, Kostelic was not considered among the favourites in the giant slalom.
The Croatian, who missed the first half of the World Cup season while recovering from three knee operations, ranks only 30th in the giant slalom on the World Cup circuit this season.
She has also never finished higher than fourth in a World Cup giant slalom event.
But, in a race often decided by hundredths of seconds, Kostelic blew away the competition, finishing an amazing 1.32 seconds ahead of nearest contender Paerson.
It was also a good result for Paerson who added the silver to the bronze she won in Wednesday's slalom.
But it was disappointment for Austrian Alexandra Meissnitzer, a former world champion and silver medallist from Nagano four years ago.
She failed to match the pace in the second run and finished in fourth.