Daniele Bosio, the Italian ambassador to Turkmenistan, was on holiday in the Philippines when he was detained, the ministry said yesterday.
He is accused of "violating a law to protect minors", according to reports in the Italian media, although it was not confirmed in what respect the law is supposed to have been breached.
The 1992 law covers a broad remit covering everything from the exploitation of child labour to the possession of images of child pornography.
The paper said the diplomat denies the allegations.