The painting is one of only two known copies of Rubens' masterpiece, depicting Salome with the head of John the Baptist, was made shortly after the original, which dates back to 1624-5. The original is thought to have been destroyed.
“In this case, copy is in no way a negative term,” said Samuel Wittwer, of the Prussian Palaces and Gardens Foundation.
“One had to have a Rubens,” he said, adding that at the time, it was the done thing to commission copies of great art.
It featured at the opening of Frederick the Great's picture gallery at the Sanssouci palace complex in Potsdam in 1764.
But after the end of World War II it was missing, stolen and taken to Russia by the Red Army as war booty.
About 18 months ago the painting turned up for sale in the US, setting off alarms at the Art Loss Register that it may have been the stolen work.
“From then on it was a thriller,” said Wittwer, describing how he took a black-and-white photograph with him to the US to see if it was what he was looking for. Tiny marks which matched were enough to show it was.
Authorities there persuaded the woman – the daughter of a Russian immigrant family - who wanted to sell it, to give it up.
“For us it is a gift,” said Wittwer.
US Ambassador Philip Murphy was at Potsdam with the director of the gallery, Hartmut Dorgerloh, on Wednesday to see the painting returned. It was just one of around 4,000 valuable artworks from the Prussian palaces thought to have gone missing in the war.