The French sister Nathalie Becquart is the first woman to be appointed to the post and will have the right to vote in the synod.
This puts her in a position to make important decisions in the Catholic Church, according to the Vatican News, the official press website of the Holy See.
The Synod gives advice to the Roman Pontiff, including many controversial topics such as divorce.
Pope Francis appointed Becquart, along with Fr. Luis Marn de San Martin as the youngest under-secretaries.
Cardinal Mario Grech, head of the synod, told Vatican News that a “door has been opened” for the possibility that more women could vote.
“During the last synods, many synodal fathers stressed the need for the whole church to reflect on the place and role of women within the church,” Grech said.
“Even Pope Francis has repeatedly emphasized the importance of women being more involved in the processes of discernment and decision – making in the church. Already in the last synods, the number of women participating as experts or auditors has increased.
“With the appointment of Sr. Nathalie Becquart and the possibility that she will participate with the right to vote, a door is open. We will then see what other steps can be taken in the future,” Grech added.