EXCLUSIVE

Ryan Seacrest must be one understanding boyfriend, because he's still with Julianne Hough ... after she left $100,000 worth of jewelry he gave her in her car ... and guess what happened?

Julianne was in Hollywood Friday night, visiting a friend in an apartment complex. Law enforcement sources tell us ... she parked her Mercedes in the lot, leaving 3 pieces of jewelry inside the car -- jewelry Ryan had given her.

A few hours later, Julianne went to her car ... and looked in horror as she saw the door wide open and the inside ransacked.

It's unclear if Julianne left the door unlocked, but there were no visible signs of forced entry.

We don't have specifics on the 3 pieces of jewelry, but we've confirmed the total value is $100k.

It'll take 10 minutes for Ryan to make enough to replace those baubles.