The philanthropist – who was married to the iconic American entertainer from 1976 until his death in 1998 – died of natural causes at home in Rancho Mirage, California, where she was surrounded by family and friends.
Barbara was a former model and a Las Vegas showgirl, but she also spent much of her life campaigning on behalf of abused children.
In 1986, she and her husband founded the Barbara Sinatra Children’s Center in Rancho Mirage, which provides therapy for children who have been abused.
John Thoresen, director of the Barbara Sinatra Children’s Center, said: “Barbara started raising funds for it in 1985 with Frank’s support.
“It opened in 1986, and since that time over 20,000 children have received beneficial therapy here.
“There are several child advocacy centres like it around the country, [but] this one is probably most recognised.”
Barbara was actively involved in the operations of the facility from the outset.
Prior to marrying Frank, Barbara had tied the knot on two previous occasions, including to Zeppo Marx, who was the youngest of the Marx Brothers comedy family.
And in 2011, Barbara insisted she had no issues with her husband remaining on good terms with his three earlier wives, including actress Mia Farrow.
She said: “A very wise French lady once said to me: ‘You never worry about old flames. You worry about new ones.'”
Barbara and Frank ultimately spent 22 years of their lives together, before the legendary singer died of a heart attack, aged 82.
Barbara – whose union with Frank was the longest lasting of the singer’s marriages – is survived by her son, Robert, his wife Hillary, and her granddaughter, Carina.