National park spokesperson Kirby-Lynn Shedlowski says park officials received a call via satellite phone on Wednesday at about 7:20 p.m. reporting a woman fading in and out of consciousness due to an apparent allergic reaction.
The woman, identified as Kathy Newman, 54, of Seattle lost consciousness and members of the rafting group initiated CPR.
Medics with the Arizona Department of Public Safety responded to the scene by helicopter and provided life support, but all attempts to revive Newman were unsuccessful and she was pronounced dead.
Officials did not say what sparked the allergic reaction.
Newman's group was on the fifth day of a seven-day commercial rafting trip down the Colorado River when the incident took place.
An investigation is being conducted by the National Park Service and the Coconino County Medical Examiner.