Lupita Nyong'o -- Dress Returned to Scene of the Crime!

Lupita Nyong'o's Oscar gown was returned to The London West Hollywood Friday afternoon where it was taken 2 days earlier ... and the thief called TMZ to tell us where it could be found and added it was a worthless fake.

A guy called us at around 2:30 PM Friday and said he had taken the dress from Lupita's hotel room after he noticed the door was ajar. He said he and others took 2 pearls off of the dress and took them to the garment district in downtown L.A. ... where they were told they weren't real.

The thief says he decided to take the dress back to the London in a garment bag that was inside a trash bag that he deposited in a 2nd floor bathroom adjacent to a fitness center.

We contacted the L.A. County Sheriff's Dept. and gave them the info. They went over to the hotel and retrieved the bag and they found a garment bag with the dress inside.

Law enforcement says they were told the Calvin Klein dress was worth $150K, but the thief says it's almost worthless.

The thief told us he returned the bag and called us because he wanted the world to know "Hollywood's fake bulls**t."