The 43-year-old star, who is on a romantic European break with boyfriend of one year, Justin Theroux, was in Rome visiting religious sites.
She wore a short grey jersey dress that contravened what the Vatican specifies as appropriate wear.
According to the Basilica’s website, the instructions for clothing when visiting the here are quite clear.
It says: ‘The Dress Code is strictly enforced at St. Peter's Basilica. No shorts, bare shoulders or miniskirts.’
While the 'Horrible Bosses' star did cover her shoulders with a grey cardigan, the short dress was in conflict with the wardrobe regulations.
The star was accompanied by a uniformed guard who followed her and Justin on their visit to the Basilica.
The couple arrived in Rome having spent a few days in Paris.