The giant blue "Modern Head" sculpture stands outside the museum in downtown Washington. It's been on loan since last year.
Smithsonian Secretary Wayne Clough said it's likely the most significant gift of art to the Smithsonian, in terms of the artwork's value.
The sculpture was created in 1989 and in 1996 was installed in Battery Park City, one block from the World Trade Center. While millions of dollars worth of American artwork was damaged or destroyed at the site, the sculpture survived the attack with only surface scratches and was temporarily used by the FBI as a message board during its investigation.
The sculpture was removed from the site on Nov. 9, 2001, and was subsequently on view at the Nassau County Museum of Art in Roslyn Harbor, N.Y., and then at the Fairchild Tropical Botanic Garden in Coral Gables, Fla., before going to D.C.