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.
Yellowstone and nearby surrounding areas are home to a growing number of grizzlies, at least 600 and some say more than 1,000. Once a rare sight, they've become an almost routine cause of tourists lining up at Yellowstone's roadsides at the height of summer season.
Their growing numbers require constant vigilance by tourists and park workers alike, said Caleb Platt, a service station manager at Canyon Village.
Platt lives most of the year in Yellowstone and said over the last eight years he has had three fairly close run-ins with grizzlies while hiking.
"When it's close and you realize it does see you, it gets the heart racing," he told The Associated Press by phone.
He said he hadn't heard about the nearby mauling.
He said he carries bear spray -- pressurized hot-pepper oil in a can -- so he's able to defend himself in case a bear gets ever too close on the trail.