The woman, 22, was found in the early hours of Saturday morning after a Christmas Party at the Millennium Madejski Hotel, which is part of the stadium complex.
She suffered serious head injuries and was pronounced dead at the scene.
Thames Valley Police Detective Inspector Phil O'Neil said: "The woman had been attending a Christmas party at the time of the incident.
"We are treating her death as unexplained and there are a number of inquiries taking place to establish the full circumstances of what happened."
The woman has not yet been formally identified although it is believed next of kin have been informed.
No arrests have been made although anyone with information has been urged to call the police on 101 or Crimestoppers on 0800555111.
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