The ban had previously applied only to St Peter's Basilica but guards manning the official entry point into the tiny, walled state begun pulling visitors aside overnight for sporting "inappropriate" attire, Italian news agency ANSA reported.
The new decree, particularly targeting those in shorts and shoulder-revealing shirts, sent several tourists trudging off to local stores to buy shawls, scarves and pairs of trousers.
Immodest clothing has been specifically barred from St Peter's for decades and Vatican officials are seemingly sick of visitors flouting the dress-code.
However, locals just nipping inside the Vatican City walls to use its pharmacy, post office and store were particularity bewildered by the clothing crackdown.
Most are accustomed to treating the Vatican like any other part of Rome, ANSA said.