The Paris prosecutor's office opened an investigation on January 24.
Ventura, 74, has been based in Paris since 2009, serving as a diplomat for Pope Francis.
The archbishop is alleged to have inappropriately touched a junior male official working at the Paris city hall, Paris deputy mayor Patrick Klugman told CNN Friday.
Allegedly, Ventura repeatedly caressed the young man’s buttocks 'in an insistent and repeated manner' during Mayor Anne Hidalgo’s New Year address.
The official, member of Mayor Hidalgo’s international relations team, was assigned to look after the archbishop during the event.
Following the alleged sexual assault, the city hall filed a complaint with the Paris prosecutor.
The Vatican said in a statement that the Holy See was aware of the investigation and that it "awaits the results." While the Vatican's embassy in Paris said it would not comment on the investigation.
The development comes after a hellish year for the Catholic Church that saw more revelations of abuse and coverups, prominent resignations and a former Vatican ambassador urging the Pope himself to step down.