Miley Cyrus -- Birthday Burglary Victim

Miley Cyrus was given the worst birthday gift ever this weekend ... her house was broken into and her stuff got jacked.

The break-in occurred Friday afternoon in Studio City - one day before her b-day - when the thief somehow gained access to Miley's pad ... and made off with some jewelry.

It's unclear how the crook (or crooks) got inside the house ... but cops say they're currently investigating how it all went down.