An Australian jeweler feared she was about to be robbed by a shady American ... who goes by the name Leo DiCaprio.

A clerk named Linda -- who works at The Family Jewels shop in Sydney -- panicked when she saw two burly men walk into her store and then stood guard at the door. Suddenly, a man whose face was concealed by a hoodie, cap and dark sunglasses entered the store. Linda sensed she was about to get robbed, so she sent a customer next door to summon the owner of The Family Jewels.

Alas ... by the time the owner arrived, Linda realized it was not a robber, but a "Titanic" star who was more than able to plunk down $1600 for a Nepalese antique necklace set with lapis lazuli and coral.

Problem was ... Leo's car was blocking traffic, so he said he'd come back to buy the piece.

And Linda had a good story at the dinner table.