A family spokesman said Sinatra -- born Barbara Blakely -- died of natural causes surrounded by family and friends at her home in Rancho Mirage.
She wed Sinatra in Rancho Mirage in July 1976. She was 49 and he was 60.
They were married 22 years, the fourth and longest marriage for the American crooner. He died of a heart attack in 1998.
In 1985, she raised money with her husband's support, to open the Barbara Sinatra Children's Center for the youngest victims of physical, sexual and emotional abuse.
"Barbara Sinatra was not a woman who just lent her name to a cause or event," her foundation said.
Trough her efforts, more than 20,000 children have received therapy to cope with abuse-related trauma at the center, it said.
Sinatra was previously married to actor Zeppo Marx, of the Marx Brothers films.
Sinatra is survived by her son Robert Oliver Marx, his wife Hillary Roberts and her granddaughter Carina Blakeley Marx.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)