A 43-year-old Mississippi woman was thrown into the air recently after she turned her back on a bison to take a photo of herself with it.
Four other tourists have been hurt in similar incidents this year, Yellowstone officials said.
Previously bison attacks had been unusual, they added.
"The (woman) said they knew they were doing something wrong but thought it was OK because other people were nearby," park spokeswoman Amy Bartlett told the Associated Press.
"People are getting way too close."
The woman and her daughter got within about 18ft (5.5m) of the animal near the Fairy Falls trailhead just outside Old Faithful. The sprawling park is mostly located in north-west Wyoming.
Someone warned them that they were too close, but the bison charged and the woman could not run away fast enough.
Park rules say people should stay at least 75ft (23m) away from the bison. The massive beasts can weigh more than 2,000 pounds (900kg) and can move extremely quickly.
The woman suffered only minor injuries, but in June a 68-year-old Georgia woman was seriously hurt after a bison gored her along a trail.
A series of injuries at Yellowstone National Park has officials warning tourists that selfies and bison don't mix.