Marcello di Finizio perched himself on the facade of St. Peter's Basilica by using mountain climbing ropes to lower himself down from an area open to the public.
The Italian appeared on TV pacing across the roof using his mobile phone late on Sunday night.
He's still there and has posted a 'selfie' on social media.
Pope Francis is due to give his Christmas address directly below where Di Finizio is.
It's the latest of five security breaches he's managed at the Vatican - the last in March.