A statement from the Holy See's press office said that Vatican prosecutors on Monday upheld the arrests of the two, who had been interrogated over the weekend.
It identified the woman as Francesca Chaouqui and the monsignor as the Rev. Lucio Angel Vallejo Balda.
The monsignor is still a Vatican employee while Chaouqui had served on a commission that had been set up by Pope Francis in 2013 as part of his drive to reform the Holy See's finances.
Vallejo Balda had also served on the commission, now defunct.
A Vatican spokesman, the Rev. Ciro Benedettini, said Vallejo Balda was being held in a jail cell in Vatican City. Chaouqui was allowed to go free because she cooperated in the probe, the Vatican said.
"In the context of judicial police investigations carried out by the Vatican gendarmerie, or police force, and begun several months ago because of the removal and lead of confidential information and documents, on Saturday and Sunday (the) two persons were summoned to be interrogated on the basis of elements and evidence that had been gathered," the Vatican statement said.
While Francis is intent on modernizing the Vatican and making its finances more transparent, the arrests were the latest confirmation that scandal and intrigue still swirl, as they have for centuries, through the largely closed world of the tiny city-state's administrative bureaucracy.
Current and past papacy efforts to clean house at the Vatican have sparked resentment and found resistance in the Holy See's entrenched bureaucracy, a perfect combination of factors to foster leaks. Read more: