42-year-old Florent Gonçalves was handed a two-year prison sentence, with one year suspended, after a court was told about his relationship with 23-year-old Emma Arbabzadeh.
Prosecutors alleged that the relationship between the two started in December 2009 in the prison west of Paris.
The prison director allegedly favoured her with frequent visits, mobile phones, SIM cards and the creation of a Facebook page, allowing them to communicate with each other, reported Le Figaro newspaper.
Inside the prison, other inmates reportedly knew about the prison head's preference for the inmate who they called "madame la directrice".
Arbabzadeh had been jailed over a notorious case known in France as the Gang des Barbares (Gang of Barbarians).
It involved the kidnapping, torture and murder of a young French Jewish man, Ilan Halimi, in 2006.
In total, 27 people were accused of the crime with the gang's leader receiving a life sentence.
Arbabzadeh was accused and convicted for having lured Halimi into the gang's lair.
Arbabzadeh told the court she had fallen in love with the director but said she didn't want to reveal any more about her private life.
The court also heard they had two sexual encounters, in an IT room, reported Le Parisien newspaper.
Gonçalves was also fined €10,000 and banned from working in the public sector again. Arbabzadeh received a one year prison sentence.