Thomas Steu and Wolfgang Kindl took silver for Austria, with Tobias Wendle and Tobias Arlt, seeking an unprecedented fourth successive gold in the event, getting bronze for Germany.
On a night of huge drama, Marcus Muller and Ansel Haugsjaa had looked on course for a chance of a first Olympic luge gold for the United States as they led after the first run, but it all went wrong near the end of their second, and they fell back to finish sixth.
After Andrea Voetter and Marion Oberhoffer had taken a surprise gold in the first Olympic women’s doubles, the atmosphere was electric around the Sliding Centre.
Rieder and Kainzwaldner were not even the top Italian pair in the competition, but, like all their compatriots who have spent so much time learning the optimum lines on their home track, they delivered an error-free first run to sit third at the halfway point.
Ahead of them, Muller and Haugsjaa, setting off on the 10th, posted a track record 52.482 seconds – three thousandths of a second ahead of Steu and Kindl, with Wendle and Arlt only fifth.
On the second run the Italians got themselves to the top of the leader board with two teams to go, then were shunted up to second as the Austrians failed to match them.
All eyes were then on the US pair of Muller and Haugsjaa, who seemed on course for glory before losing their way in the latter stages to slip right back, two tenths behind the winners.
Italy's double gold put the brakes on Germany’s usual charge. The sport’s dominant nation, who swept all four events in Beijing, still have the two singles titles to their name but though they had been hot favourites to win a fourth successive team relay, the Italians will be making them think again.