Monsignor Lucio Angel Vallejo Balda, 54, who served on a special commission set up by Pope Francis to advise him on economic reform within the Vatican, was arrested along with a second member of the commission, Francesca Chaouqui, who has been dubbed a "sex bomb".
The arrests were part of a several months-long investigation into the "misappropriation and disclosure of classified documents and information".
They 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.
Chaouqui's appointment to the economic committee, which was handpicked by the pope, caused no little embarrassment in 2013 when it emerged the woman dubbed a "sex bomb" by the Italian media had been highly critical of the Vatican on Twitter.
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Vatican expert Sandro Andrea Tornielli said today it was Vallejo Balda who got Chaouqui a place on the committee.
A furious 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.
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.
Butler Paolo Gabriele was sentenced to 18 months in prison for stealing secret papal memos but pardoned by Pope Benedict, who nonetheless banished the once loyal servant from the Vatican for good.
"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 Vatican statement said.