A ward staff member made the discovery at about 1 am during a regular patrol. Attempts to resuscitate the woman failed and she was certified dead about two hours later.
The woman had been admitted to an orthopaedics ward last Wednesday, following a fall. She had been in a stable condition.
The hospital said it was deeply saddened by the incident, and that next of kin had been informed. It added it would fully cooperate with the police investigation and that the incident had also been reported to the Hospital Authority and to the coroner.
Tuen Mun Hospital said it too would investigate and submit a report in eight weeks.