The 38-year old, who was World Cup champion in 2005, was lying second behind Italy's double Olympic champion Armin Zöggeler after the first run.
But after Demtschenko had laid down a powerful second run of 50.165 seconds, Zöggeler had no response and was forced to settle for second-best, 0.050 secs behind the Russian.
Germany's Felix Loch, who also relegated Zöggeler to silver last week in Altenberg, collected bronze in Norway, 0.318 secs behind Demtschenko.
Having won the first two World Cups of the season, defending champion Zöggeler remains at the top of the standings on 370 with Loch second on 290 and Demtschenko up to third on 279.
Meanwhile, Tatjana Hüfner continued Germany's dominance in the women's competition although American Erin Hamlin reached the podium for her first World Cup medal.
Germany has won every World Cup event since 1997 and defending World Cup champion Hüfner came from third after the first run to pick up her third gold of the season, finishing in 1:37.579.
Hüfner's compatriot Natalie Geisenberger came home 0.016 secs behind for silver but there was a feeling of what might have been for Hamlin after her bronze.
The American, who won the World Championships earlier in the year, was lying first after the first run but after producing the fourth quickest time in her second run, slipped to third, 0.037 secs behind Hüfner.
But Hamlin refused to be downbeat about her performance in Lillehammer.
"After leading in the first run it's natural to be a little bit disappointed but I'm so happy about coming third - my first World Cup medal feels great," said the 23-year old.
"I was a bit nervous beforehand, even though my training in Norway was going well so it's nice to show that I can do it in the World Cups."