The attack was the first fatal bear mauling in the park since 1986.
"In an apparent attempt to defend a perceived threat to her cubs, the bear attacked and fatally wounded the man," the park stated in a statement. "Another group of hikers nearby heard the victim's wife crying out for help, and used a cell phone to call 911."
Investigators have been interviewing the woman about the bear attack, which took place on the Wapiti Lake Trail, close to Canyon Village and near the middle of the park, park spokesman Al Nash said. Park officials haven't taken any action against the bear, he said.
"This is complicated because of where this has occurred," Nash said. "It is in the backcountry of the park, and we have access challenges and limited communication."
Park officials were working to clear the area of other hikers. All trails and backcountry campsites in the area have been closed and a warning sign has been posted on the trailhead.
Nash said authorities aren't prepared to release the man's name, age or hometown. He said more details probably won't be released until Thursday, after the man's family is notified.