The coroner Sir John Goldring told the remaining jury members there was a "medical basis" for the woman being discharged.
Six women and three men will continue to consider how and why 96 Liverpool fans died in Britain's worst sporting disaster on 15 April 1989.
The jury was sent out on Wednesday after more than two years of evidence.
The hearings, which are the longest running inquests in British legal history, began on 31 March 2014 with the maximum of 11 jurors.
That number was reduced to 10 in February last year when a juror was discharged for "wholly exceptional medical reasons".
The minimum number that can hear evidence at an inquest is seven.
A member of the Hillsborough inquests jury has been discharged on the first full day of deliberations.