The 32-year-old — whose other success this season came in Val Gardena in December — recorded his fifth career victory as he edged out Frenchman Johan Clarey, while Austria's recently-crowned Olympic champion Matthias Mayer was third.
For Mayer it was his first-ever podium finish in a World Cup event, his best previous finish being sixth.
While home favourite Aksel Lund Svindal could not reward the crowd with victory, his sixth place was good enough for him to leapfrog Austrian slalom specialist Marcel Hirscher in the race for the overall World Cup title.
The 31-year-old Norwegian — who had a disappointing Olympics coming away with no medals from his three events — leads Hirscher by 27 points.
"That is far too little because the next few races favour Marcel," said Svindal, who after downhill was guaranteed the discipline's crystal globe.
Svindal, who also sees France's slalom Olympic bronze medalist Alexis Pinturault as a danger in the overall race even though he trails hm by 208 points, could also end the weekend with another crystal globe when he competes in the super-G and also give himself some extra breathing space over Hirscher.
Guay, who won the super-G at Kvitfjell in 2010, was by far the fastest on the lower part of the course where he reached a speed of 142 kilometres per hour.
"I would like to thank my kit man, who prepares the skis to be race-ready, as in the last seconds of the race I was extremely fast," said Guay.
"It was a good race by me, the conditions like yesterday were difficult with a very soft piste in the middle.
"Yesterday, it was not an advantage to start with a high number.
"Today I succeeded in benefiting from my starting spot."