A husband and wife couple were hiking on the Wapiti Lake Trail when they encountered a grizzly bear sow with cubs. The bear attacked and fatally wounded the man, according to a park news release.
A group hiking nearby heard the victim's wife calling out for help and they called 911 on a cellphone.
Yellowstone officials closed all trails and backcountry campsites in the area indefinitely while the incident was under investigation. A warning sign was posted at the trailhead.
The Wapiti Lake is known for its significant bear activity, according to park officials. Park officials did not take action against the bear.
"This is complicated because of where this has occurred. It is in the backcountry of the park, and we have access challenges and limited communication," a park spokesman said.
The man's name and home town were not released pending notification of next of kin.
Wednesday's attack is the first reported bear encounter of the 2011 season, according to the park's news release.
Attacks by bears are extremely rare. No visitors were injured by bears in the park in 2010.