Agence France-Presse reports that the dress, worn by Callas at a 1957 concert performance in Athens, was returned to the center on June 1, wrapped inside an old videotape case.
"We did not fully open the package because the police must examine it first, but the robe seems to be unharmed beyond the deterioration the silk has already suffered because of its age," a spokesman for the center told AFP.
The theft occurred on May 25, during a three-day break for the Pentecost holiday. The exhibition of memorabilia, including gowns, jewelry and accessories, was part of a series of events (including an international opera competition) held in Greece this year to honor the soprano.
Callas was born to Greek immigrant parents in New York and became a Greek citizen in 1966 at the age of 43, at the end of her relationship with Greek shipping magnate Aristotle Onassis. She died 11 years later in Paris.