It's the first time in more than a half-century that the Wittelsbach gem has been on public display, and it will remain at the museum until Aug. 1.
Both diamonds first became known in the 17th century and while they are together scientists plan tests in hopes of learning if they came from the same mine in India where the Hope is known to have originated.
The Wittelsbach Diamond was first reported in the 1660s when Philip IV of Spain gave it to his daughter, who was to marry Emperor Leopold I of Austria.
In 1722 it became the property of the Wittelsbachs, the ruling family of Bavaria. It disappeared after World War I, resurfacing in Belgium in 1951, and it was auctioned last year by Christie's in London for more than $24 million. It was acquired by jeweler Laurence Graff, chairman of Graff Diamonds International Ltd.
Smithsonian officials have declined to estimate a value for the Hope Diamond, simply referring to it as priceless.
"To have two of the world's most historical stones - the Wittelsbach-Graff and the Hope Diamond - displayed together, is a testament to the stones' history and importance," Graff said in a statement. "I believe the diamond's appearance at the Smithsonian will represent another significant chapter in its remarkable history."