The 200-hectare fire is burning near Excelsior Creek in the Maligne Valley.
People are being cleared from the valley, the Skyline trail and the area around Maligne Lake. The Maligne Lake Road is closed.
"Most of our efforts have been focused on evacuating people from the valley," said Greg Fenton, Jasper field unit superintendent for Parks Canada. "Both the day-use area around Maligne Lake ... and then starting evacuations from backcountry campgrounds."
Fire is not uncommon in the national park and is a natural part of the ecosystem, Fenton said.
"The forests that we have in the Jasper National Park are the remnants of historic fires that burned from one side of the valley to the other. But we're not in that kind of a situation and we certainly don't anticipate that we will see fire growth like that."
The fire was reported at about 3:30 p.m. Thursday and is moving south through the valley, away from the town itself. Crews are working to contain the blaze.
"This was a lightning-caused fire," Fenton said.
All park facilities remain open, including campgrounds.