The victim was killed on Wednesday about a mile from a West Glacier campground.
The authorities say armed police officers are searching for the bear but gave no further details.
Bear attacks are rare in the area with only 10 deaths reported since the park was created in 1910.
The dead cyclist was a 38-year-old officer with the US Forest Service, Flathead County Sheriff Chuck Curry told Daily Inter Lake.com.
Officials say the grizzly confronted two cyclists on a trail after they disturbed it.
The dead rider was taken off his saddle by the bear and the other rode off to get help.
The US Fish and Wildlife Service recently announced plans to remove grizzly bears from the threatened species list.
Who, What, Why: What should you do if you encounter a bear?
A grizzly bear has attacked and killed a cyclist just outside the Glacier National Park in the northern US state of Montana, police say.