Guay, from Mt. Tremblant, Que., clocked one minute 56.65 seconds down a Saslong course lined with huge jumps and filled with tricky terrain, making up time over the last few gates to edge Kjetil Jansrud of Norway by 0.12 seconds.
"It is sweet," Guay said. "I skied really well and took the chances necessary."
It was the fourth career World Cup victory for Guay, who also won gold in downhill at the 2011 world championships in Garmisch-Partenkirchen, Germany.
Johan Clarey of France finished third, 0.24 back, and overall World Cup leader Aksel Lund Svindal was fourth, 0.29 behind.
Bode Miller placed fifth, 0.39 behind, for his best finish in a speed event this season.
Guay was one of three Canadians in the top 10, with Manuel Osborne-Paradis of North Vancouver, B.C., charging into seventh and Calgary's Jan Hudec finishing ninth.
Miller took last season off to recover from left knee surgery and he is improving race by race as he prepares for the Sochi Olympics in February.
Miller would have made the podium if he was just a bit faster on the flats and said getting his equipment dialed in after so much time off was a big factor.
"There's so many details, you never know what it is that's really causing the problem," the 36-year-old two-time overall World Cup winner said. "The fact is we just don't have the whole thing together right now. We're close, and we're getting there."
Miller made up more than half a second on the technical lower section.
"On the top I didn't feel like I was going fast," Miller said. "I took a little bit of risk in some parts and pulled off some decent turns and I skied pretty well but it's one of those courses where there's not enough to it to bring it back in with my technical ability."
Guay thought he had won this race last year until a drastic weather change helped later starters on a shortened course, with Steven Nyman of the United States taking the win and Rok Perko of Slovenia claiming second, relegating Guay to third.
"I had a great run last year," Guay said. "In my mind I skied the best that I could have with my start number and not to take anything away from Nyman and Perko but the conditions do change and that's the reality of our sport.
"I know that ski racing is like that," Guay added. "I've been on the good side of the hundredths and I've benefited from good weather sometimes. So you take it when you can and you forget it when you're on the other side."
While it wasn't quite as drastic as last year, when winds played a role, changing light once again made a big impact. Guay, Svindal and Miller had to deal with less light on the gliding section on top but then all three racers excelled on the more technical section of the course.
The highlights of the Saslong are the camel bumps, which launch skiers into the air for nearly 80 metres (90 yards), and the twisty and technical ciaslat section — two areas that come one right after another midway down. There are also numerous little bumps and jumps, making for constant air time.
"A lot of times Val Gardena goes under radar with Wengen and Kitzbuehel getting so much attention but I think it's one of most fun courses on the World Cup," Guay said.
Guay's performance was all the more impressive considering that he had left-knee surgery in the off-season and missed most of preseason training.
Jansrud also had left-knee surgery after a crash in the super-G at last season's world championships in Schladming, Austria.
Jansrud took silver in giant slalom at the 2010 Vancouver Olympics and Saturday's result matched his best downhill finish from March, 2012, on home snow in Kvitfjell.
Clarey also finished third in this race four years ago for his only other career podium. He stood first when the 2011 race was abandoned beause of strong winds after 21 racers had completed their runs.
"It's unbelievable for me," Clarey said. "I almost quit skiing five months ago after my back injury."
In the overall standings, Svindal moved 175 points ahead of Marcel Hirscher, who does not race downhill. Svindal also leads the downhill standings, 53 points ahead of Guay.
Nyman was an early starter and made a big mistake midway down and had to make an acrobatic recovery to avoid injury. But his race ended there. Brice Roger and David Poisson of France also fell.
Poisson slammed into the safety netting at full speed but eventually got up and skied down.
The circuit moves over the Gardena pass for a giant slalom Sunday in Alta Badia, where Ted Ligety will attempt to duplicate his dominant win from last season.