The 24-year-old, who also claimed gold in the downhill and team combined, once again left nothing in the tank as he went full throttle down the bumpy and winding course to win by 0.13 seconds from American Ryan Cochran-Siegle.
Marco Odermatt, who had been tipped to win multiple golds in the build-up to the Games, found himself eclipsed by his younger teammate again and had to settle for bronze to add to the silver medal he won in the men's combined event on Monday.
Not since French great Jean-Claude Killy in Grenoble in 1968 has a male Alpine skier won three golds at the same Olympics -- Killy doing it in downhill, slalom and giant slalom.
Seventh man out of the start gate, Von Allmen produced a typically aggressive run on a course that was 800 metres shorter than the one on which he won Saturday's downhill.
At times he was on the edge of control but it paid off as no other skier could match his speed.
Von Allmen looked almost apologetic when Odermatt crossed the line with a slower time, but beamed as the late runners came down, eventually holding up three fingers for the camera.
Cochran-Siegle took advantage of the better early conditions on a mild morning in Bormio, repeating the silver medal he took in super-G at the 2022 Beijing Games.
There was heartbreak for Italian veteran Stelvio King Dominik Paris. He lost a ski early in his run but thankfully managed to avoid injury as he slid out.
Fellow Italian Giovanni Franzoni, second in the downhill, also suffered disappointment as he could only finish sixth.