The downhill section was originally scheduled to start the combined event on Friday morning, but had to be cancelled after just
one skier due to strong wind and was moved to Saturday.
Kostelic had missed Wednesday's downhill because of a high puls rate, but took part in Friday's slalom section of the combined despite a fever.
Austria's Marlies Schild, who led after Friday's two slalom runs, took silver medal while Anja Paerson of Sweden missed out on her first Olympic gold again, getting bronze in 2:51.63 minutes.
Kostelic, who won three gold medals at Salt Lake City in 2002, including the combined, overturned Schild's 0.46 second lead from the slalom and won by 0.50 seconds.
Janica's latest victory came only a few days after her brother Ivica won silver in the men's combined event.
Kostelic was already part of the the Olympic record books having become at Salt Lake the first Alpine skier to win four medals at
one Games.
The Croatian was one of four women to have won three Olympic golds along with Switzerland's Vreni Schneider, Italian
Deborah Compagnoni and German Katja Seizinger.