Monsignor Lucio Angel Vallejo Balda, who served on a special commission set up by Pope Francis to advise him on economic reform within the Vatican, was arrested as part of an investigation into the "misappropriation and disclosure of classified documents and information".
The arrest followed Italian media reports at the weekend that Vatican police were investigating the attempted theft of a laptop belonging to Libero Milone, the head of the city state's new finance office.
Monseñor Lucio Ángel Vallejo Balda, está acusado de filtrar documentos secretos. pic.twitter.com/zlOhOReN2R— Alfredo Martinez (@alfredomdoza) November 2, 2015
Balda was hauled in for questioning on Saturday along with a second member of the commission, Francesca Chaouquil, after a probe lasting several months, a Vatican statement said.
Both were arrested but Chaouqui was released by Vatican prosecutor Roberto Zannotti on Monday because she agreed to collaborate with investigators and was not considered a flight risk.
The Vatican also denounced the expected publication this week of two new books believed to be based on leaks from hackers releasing information regarding the famously murky world of its finances.
And it referred to the last time employees aired the centuries-old institution's dirty laundry.
In 2012, Pope Benedict XVI's butler engineered a series of leaks that revealed fierce infighting in the highest echelons of the Catholic Church and allegations of serious fraud in the running of the city state.
"As far as the books announced for the coming days go, it is clear that this time too, just as in the past, they are the fruit of a serious betrayal of the pope's trust," the statement said.