It was found in a bathroom of the same Hollywood hotel where it was taken.
Police were tipped off by gossip site TMZ, who said they were contacted by a man claiming to be the thief.
He said he had returned the dress after learning the pearls studding the dress were fake.
The custom Calvin Klein-designed gown is coated with 6,000 natural pearls, but the purported thief said he took two of them for testing and were told they were not real.
Detectives are working with the owners of the dress to confirm if it is the dress.
But Michael White of the Los Angeles County Sheriff's Department said he believed it was. No arrests have been made.
''Whether the jewels on the dress are fake or real... we still have a burglary and we still have a grand theft,'' Mr White said.
Nyong'o won best supporting actress in 2014 for 12 Years a Slave and was a presenter at Sunday's ceremony.
Speaking about the dress's design, the 31-year-old said: "We talked about it being fluid and liquid. I wanted it to be an homage to the sea."
[H/T: BBC]
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