A 45-year-old woman filed the case against the Swiss academic, according to media reports. The woman accused Ramadan of multiple sexual assaults in 2013 and 2014.
The 55-year-old Swiss national had previously faced two charges of raping two women in French hotels in 2009 and 2012. Following the accusations, he was ordered to remain in custody in France.
A preliminary probe was opened in October 2017 after the two women filed complaints.
Ramadan was arrested last month by French police and after 48 hours of questioning, a judge ordered him to be held in custody.
The Geneva-born professor has denied the accusations that forced him to take leave of absence from Oxford since November 2017.
Ramadan is the grandson of Hassan al-Banna, the Egyptian imam who founded the Muslim Brotherhood in Egypt in the 1920s. Ramadan has written several books on Islam and the Western world which, according to experts, project a moderate and tolerant view of Islam.