According to the Press-Enterprise, the woman was in cardiac arrest when firefighters responded to the scene just before 11:40 a.m.
The woman was treated by first responders, but later died at a hospital. Her name or age have not been released.
It's unknown how far she fell.
The historic hotel is known for its rotunda, which features a circular staircase.
Riverside police say there is no evidence of foul play involved.
Further details about the incident were unavailable.