Staff of the three-time Oscar winner contacted the FBI after it posted an alert about the painting, Russian Schoolroom, on the Internet last week. Spielberg, 60, is an avid collector of paintings by Rockwell, who is famous for his portrayals of innocent American life.
Now valued at US$700,000, the picture was first stolen from a gallery in St Louis, Missouri, in June 1973. Spielberg purchased the work from a legitimate art dealer 16 years later.
"They were gracious and responsible to come forward with that information," Assistant Special Agent Tom Noble told ABC News Monday.
Art Loss Register staff members mostly manage the computer archive, which the company says is immune to hackers, and preserve the many paper files for future court cases.
The company has another 25 employees at satellite offices in New York; Amsterdam, Netherlands; Cologne, Germany; New Delhi, and Bath, England.
The Art Loss Register was created 1991, taking over and expanding a database that had been operated since the 1970s by the International Foundation for Art Research in New York.