Hancock and a crowd of people approached within 10 yards of the bison while walking along a boardwalk.
Park officials said that, at one point, people in the group were closer than 15ft from the bison. The animal then immediately left the area, park officials said.
Hancock sustained a hip injury, but was in good condition after being transported to a nearby hospital, the park service said.More news: Donald Trump says 'looking forward' to resolving trade disputes at G7
The park service said Wednesday's incident remains under investigation.
There have been two bison attacks so far in 2018.More news: Trump says he's considering pardon for Muhammad Ali
This incident is being investigated by Park staff; there is no word yet on whether the woman will be cited.
Hancock is the second person to be attacked by a bison this year, and the third hurt by wildlife in less than a week.
Rangers said: "Animals in Yellowstone are wild and unpredictable, no matter how calm they appear to be". There was one incident in 2017 and five in 2015.More news: Madison Keys edges past Yulia Putintseva to reach semi-finals
Park officials not that it's very common for cow elk to aggressively defend newborn calves and hide them near buildings and cars. The statement noted that people should keep a distance of roughly 75 feet at least from certain animals including bison and elk, and stay even further away from bears and wolves. "When an animal is near a trail, boardwalk, parking lot, or in a developed area, give it space".
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