Former US first lady Nancy Reagan died of congestive heart failure at the age of 94.
The former actress died at her Los Angeles home, a spokeswoman for the Reagan presidential library said.
"I am saddened by the passing of my step mother Nancy Reagan...She is once again with the man she loved.God Bless...," stepson Michale Reagan tweeted.
Nancy was fiercely supportive of President Raegan, who died in 2004 after a prolonged struggle with Alzheimer's disease.
Paying tributes, President Barack Obama and first lady Michelle Obama said she redefined the role of first lady.
"Nancy Reagan once wrote that nothing could prepare you for living in the White House," the Obamas wrote in a joint statement on Sunday. "She was right, of course. But we had a head start, because we we were fortunate to benefit from her proud example, and her warm and generous advice."
Nancy Raegan was a prominent Hollywood actress during the 1940s and 1950s and was known Nancy Davis before she married the would-be president in 1952.