The reigning overall World Cup champion won both runs and destroyed the field in the night's second leg to win in a combined time of one minute and 42.50 seconds.
The race was marred by the death of a 70-year-old steward, who suffered a heart attack during the first run.
Hirscher, who finished on the podium in all seven technical races held this winter, left second-placed German Felix Neureuther 1.67 seconds adrift.
His consistency and his second win of the season, after success in the giant slalom at Val d'Isere 10 days ago, lifted the Austrian to second in the overall World Cup standings, 54 points behind Norway's Aksel Lund Svindal.
"I gave it more than 100 per cent. All in all, it was an awesome second run," said Hirscher.
"Now I'm looking forward to Christmas... I can devote some time to my girlfriend," he added.
While the speed specialists will resume skiing with a downhill in Bormio on December 29, Hirscher will not race before New Year's Day at a city event in Munich.
"The World Cup remains my goal. I won it last year almost to everybody's surprise and I'm determined to confirm this season," said Hirscher.
Japan's Naoki Yuasa achieved his first career podium after climbing from 26th to third with a blistering second run.
"It's a night I'll never forget," said the Japanese, who was all the more surprised by his feat as he finished the race lying on the snow after injuring a disc on the way down.
Hirscher's victory meant he added his name to the Madonna roll of honour with the greatest slalom specialists in the sport's history, from Ingemar Stenmark to Alberto Tomba, by having also won World Cup races in the Italian resort.
"It's a great honour to be on this list," he said.
The race also saw the return to the World Cup of slalom world champion Jean-Baptiste Grange, who had been sidelined since a knee operation in March.
The Frenchman skied two solid runs and finished 21st.