Police said that the man, from Hungary, died an hour after being hit by the falling rock. The tragic incident happened as the Red Bull Hare Scramble, the final race of the event, was in full swing.
The man was watching from a wooded area on a hillside when a rock fell on him from a height of 300 metres, according to a report from the ORF.
Eyewitnesses said they tried to warn him as they saw the rock falling but it was too late. A rescue helicopter was called and paramedics tried to resuscitate him for an hour but had to pronounce him dead at the scene.
It's unclear whether the falling rock was triggered by one of the competing motorcyclists. Police will be examining mobile phone footage of the scene, taken by spectators.
Despite the accident, the race continued, with five riders finishing and four of them claiming joint first place. They were unaware of the tragedy that had occurred.