Nathalie Becquart will have voting rights in the body that advises the Pope on controversial topics.
The Pope has appointed a woman as an undersecretary to the Synod of Bishops for the first time.
Sister Nathalie Becquart, who is from France, will have voting rights in the body, which advises the pontiff and debates some of the most controversial issues in the Roman Catholic Church.
Ms Becquart has worked with the synod as a consultant since 2019.
The body's secretary-general, Cardinal Mario Grech, said the appointment showed that "a door has opened".
He noted that the decision reflects the Pope's desire "for a greater participation of women in the process of discernment and decision-making in the church".
The BBC's John McManus says the move is not a precursor to ordaining women as priests, although some opponents may regard it as a further step in that direction.
I am a Protestant. Can someone explain what is going on here?