Wildlife officials did not release the identities or ages of the victims. A response team was being sent to piece together what happened.
"We don't know if it was one bear, two bears, a black bear or grizzly bear," Montana Fish, Wildlife and Parks spokesman Ron Aasheim said. "Obviously, the bear's gone now. Will it come back tonight? That's the question."
Authorities were setting traps and seeking bear hair, saliva and droppings while measuring the bite wounds of victims to determine the type and number of bears involved in the attack.
Park County dispatchers took a 911 call early Wednesday from a male reporting that a bear had bitten his ankle and was tearing up tents, Aasheim said. Dispatchers got two more calls, including one from a man who said a bear bit the leg of his daughter's boyfriend.
At 3:50 a.m., park officials went through the campground to advise campers to get into their cars. A half-hour later, the dead male was discovered at a campsite. Authorities then evacuated the campground, sending campers to nearby hotels.
It was not immediately clear how many people were in the campground at the time.
The same campground was the site of a 2008 attack in which a grizzly bear bit and injured a man sleeping in a tent. A young adult female grizzly was captured in a trap four days after the attack and transported to a bear research center at Washington State University in Pullman.
The 10-acre Soda Butte campground has 27 sites for tents and recreational vehicles in Gallatin National Forest, some five miles from the northeastern entrance of Yellowstone National Park.
It is located just off the mountainous Beartooth Highway about 125 miles southwest of Billings.
"It is a populated area for bears, not just grizzly bears but black bears," Gallatin National Forest spokeswoman Marna Daley said.
The campground, which is run by the U.S. Forest Service, has been closed, as well as two other nearby campgrounds, Daley said. Forest Service officials will consider closing more campgrounds after consulting with state wildlife officials leading the investigation, she said.