Haemmerle crossed the finish line 0.03 seconds ahead of Canada's Eliot Grondin, who had to settle for silver again. Haemmerle's teammate, Jakob Dusek, claimed the bronze.
All three battled through qualifying rounds to face off in the finals on the the 1.1 km (0.7 mile) course in the Alpine town of Livigno. Light snow fell during much of the contest.
At the medal ceremony, the 32-year-old Haemmerle sang along to the Austrian national anthem, closed his eyes and tilted his head toward the sky as he clutched his medal.
Snowboard cross features high-speed races between four riders down a winding course filled with bumps and jumps.
The sport is known for being unpredictable with competitors often knocking into each other and crashing. Two German riders tumbled out of contention during the quarter-finals.
At Beijing 2022, Haemmerle outpaced Grondin by 0.02 seconds, and the pair have had an ongoing rivalry on the snow.
"We came here for gold, but at the same time, I rode as good as I could today," the 24-year-old Grondin said. "I had fun. Silver it is, and I can be happy with my week."
Reigning bronze medallist Omar Visintin of Italy, 36, was knocked out in the first elimination round.
Brothers Aidan and Jonas Chollet of France had shots at the podium. Aidan, 21, landed fourth and Jonas, 17, was sixth.
Jonas said he tried to offer encouraging words to his older brother after the event. Aidan led the last race until the final bend and then missed out on the medals.
"He gave it his all," Jonas said. "That's what sport is all about, that's what's beautiful about it, and that's what's hard about it too."
Nick Baumgartner, at 44 the event's oldest rider, finished in seventh spot, an improvement from his 10th place finish four years ago. He is competing in his fifth Olympics.
American rider Nathan Pare was disqualified in the quarter-finals after a jury determined he made illegal contact with the board of Spain's Lucas Eguibar Breton, who failed to advance.
"I'm just really angry because everybody knows that these things are illegal movements," Eguibar Breton said. "This is not the right thing to race."
Pare said he disagreed with the ruling and that he was trying to make a legal pass.