The 60-year-old woman, reportedly Linda McDonagh, from Offerton, Stockport, passed away on January 14.
She was thought to have been among a number of patients who suffered hypoglycaemic episodes between June and July 2011 after being given contaminated saline.
Police have been investigating the deaths of four other patients in the long-running inquiry.
The latest victim was one of 21 patients believed to have been deliberately poisoned.
Another 21 cases of patients who suffered hypoglycaemic episodes are not being treated as suspicious.
A spokesman for Greater Manchester Police said: “We have been made aware of the death of a 60-year-old woman at Stepping Hill Hospital.
“The coroner has been made aware and further tests are ongoing.”
Tracey Arden, 44, Arnold Lancaster, 71, and Derek Weaver, 83, died shortly after experiencing similar hypoglycaemic episodes to the 60-year-old woman.
All the deaths took place on wards A1, A2 or A3 – acute care wards caring for seriously ill patients.
Police are looking into whether the alleged poisoning of William Dickson, 82, at the hospital during the summer was also a factor in his death.
Mr Chua was initially arrested on suspicion of tampering with medical records and was then re-arrested on suspicion of three counts of murder and 18 counts of causing grievous bodily harm.
He was quizzed over the contamination of saline drips which caused patients’ blood sugar to fall.