Parks officials received several reports of a cougar sighting on the Lakeshore trail and sent officers to investigate. On their way back, they received additional calls from hikers who had encountered the cougar on the trail.
The hikers said the animal behaved aggressively and grabbed a teenage girl who was part of the hiking group.
The girl sustained minor injuries.
The cougar was located south of the town site and was destroyed by the resource officers.
Dennis Madsen, a resource conservation manager with the park, said the cougar's behaviour is unusual as they usually avoid humans, especially in groups.
"We don't think it had any kittens. It didn't appear to be lactating so (there's) nothing to indicate it was an unhealthy cougar or having any difficulty feeding itself, said Madsen.
"Why it suddenly chose to behave in such an unusual manner is something we may not know,"
An autopsy will be performed on the cougar and officials say it might shed some light on the cougar's behaviour.
Park users are warned to keep children close, travel in groups, carry bear spray and keep pets leashed.
Waterton Lakes National Park is located in the southwest corner of the province.