The teenage girl, who has yet to be publicly identified, was riding with her family inside the park around 7 p.m. when a rockslide began.
Rocks as large as 12 inches wide fell from the neighboring cliffside, crashing into the car's rear windshield. In addition to killing the 14-year-old, the accident left four more passengers injured, including both parents and two other children.
The rockslide occurred on Going-to-the-Sun Road, a popular sightseeing route within the park. Cars quickly began piling up at the scene.
"I could clearly hear a woman crying frantically, screaming, you know, 'Please help her. Someone please just help her,'" witness Pat Cummings told CBS News. "If there had been more cars probably closer together, I do think it could have been worse."
Paramedics arrived shortly after the accident. However, the girl was reportedly too unstable to be airlifted. She died during the ambulance ride to the hospital.
The family was visiting from Utah. Authorities said they will release the victim's name once they are able to properly notify her loved ones.
"The park extends its deepest condolences to the girl’s family, and thanks its partner emergency care providers for the significant response," the park said in a statement to ABC News. "The family asks for privacy at this time."
All four surviving victims were taken to a nearby hospital. The two adults suffered "significant bruises," while the children's injuries were reportedly more minor.
This is the first time a person has died from a rockfall on Going-to-the-Sun Road since 1996, park officials said. However, this is not the first tragic incident to occur at a national park this summer.
Earlier this month, a 21-year-old Romanian tourist slipped and fell to his death while hiking at Yosemite National Park, in California. His death led officials to remind visitors about the dangers of straying from the park's marked paths.
In July, a 9-year-old girl was thrown into the air by a bison when visiting Yellowstone National Park, in Wyoming. She survived the incident and was released from the hospital shortly after.
Glacier National Park, which was the nation's 10th-most-visited national park in 2018, had nearly three million visitors last year.