The freak accident occurred at the weekend as the man was pedalling with a colleague in Winikon near Brügelwald, cantonal police said.
At around 2pm on Saturday the trunk of a conifer cracked a few metres above ground level from natural causes and toppled into the path of the cyclists, police said in a statement.
The victim was hit by falling pieces of the tree and was so badly injured that he died on the spot, the statement said.
Investigators were initially unaware of how the tree came to fall.
Police spokesman Kurt Graf told Blick.ch that the fir tree appeared to be normal and there was no evidence ahead of time that it was liable to fall.
There were no strong wind or storm conditions at the time, Graf told the online news site.