Speaking through an interpreter, the 44-year-old woman told authorities she had been groped by her former employer from July 2010 to October 2011, the source said. She said she was fired shortly afterward.
Bahrain's ambassador to France is Nasser Al Belooshi. His embassy in Paris could not be reached for comment Tuesday.
The incidents allegedly took place at the ambassador's home in the chic Paris suburb of Neuilly.
The plaintiff initially accused her employer of raping her on more than one occasion but later told investigators she had thwarted those attempts.
The alleged victim also said the ambassador's son had threatened her with a gun in September 2010.
The initial enquiry is to determine the veracity of the allegations. It could lead to charges, the appointment of an investigating judge, or no further action.
On Tuesday, the French foreign ministry said it would assist authorities with their investigation.
Prosecutors could if necessary ask the French foreign ministry to ask Bahrain to agree to the lifting of Belooshi's immunity.