A grizzly bear has killed a man who was hiking with his wife in Yellowstone national park after the couple apparently surprised the female bear and its cubs.

It was the park's first fatal grizzly mauling since 1986, but the third in the Yellowstone region in just over a year.

The attack happened on Wednesday morning, two days after a peak weekend for tourism in the park, on a trail close to Canyon Village near the middle of Yellowstone.

Park officials said the bear attacked to defend against a perceived threat. The wife of the 57-year-old victim called for help on her cell phone and other hikers in the area responded.

A Yellowstone spokesman, Al Nash, said the couple saw the bear twice on their hike. The second time, the grizzly ran at them and the man told his wife to run.

The woman told park officials she didn't see the bear attack her husband. When it went for her she dropped to the ground, Nash said. The grizzly lifted her off the ground by the day pack she was wearing, then dropped her.

The woman did not seek medical attention, Nash said. Authorities would not identify the couple until the man's family could be notified.

Park officials cleared the area of people after the attack. All trails and backcountry campsites in the area were closed and a warning sign was posted on the trailhead.

"It is in the backcountry of the park, and we have access challenges and limited communication," Nash said.

Grizzlies in the Yellowstone region have caused growing problems as the number of tourists grows in an area of geysers and sweeping mountain vistas.

A grizzly, which had just been released after being trapped and tranquillised for a study, killed an Illinois man hiking outside Yellowstone's east gate in June 2010. Last July, a grizzly killed a Michigan man and injured two others in a camp ground near Cooke City, north-east of the park.

Yellowstone and nearby surrounding areas are home at least 600 grizzlies. Once rarely seen, grizzlies have become an almost routine cause of curious tourists lining up at Yellowstone's roadsides in the summer season.

Tourists have been visiting Yellowstone in record numbers: 3.6 million came last year, up 10% from the record set in 2009.

A spokesman for the Wyoming Travel and Tourism state agency doubted the attack would deter people from visiting Yellowstone.

"What has happened here hasn't happened for a quarter century," Chuck Coon said. "It is very sad, though, and I'm very sorry to hear of it."

Caleb Platt, a service station manager at Canyon Village, said he had had a handful of encounters with grizzlies while hiking in the park and tourists should be vigilant.

"When it's close and you realise it does see you, it gets the heart racing," he said.

Park officials have warned park visitors to stay on designated trails, hike in groups of three or more, keep alert for bears and make a noise in places where a grizzly could be lurking out of sight.