RCMP say the 37-year-old mother of two young children was vacationing with her family on Friday near Canal Flats, 80 kilometres north of Cranbrook.
RCMP Cpl. Tom Brannigan said the family was vacationing in Kimberley and driving north on Highway 93-95 when a large rock flew off the tire of a passing southbound truck.
The roughly 30-centimetre boulder smashed through the windshield of the family's Subaru, hitting the unidentified woman in the head and killing her instantly.
Brannigan said the woman was a passenger in the car and that her kids, aged 9 months and two years, were sitting in the back seat.
"It's like being hit by lightning, it's just a freak accident," he said.
Investigators believe the rock may have been wedged between the tandem wheels of the southbound trailer.
Police have spoken to the driver of the big rig and are continuing an investigation, but say charges have not been laid. (CHBZ)