Details were not immediately available and Keaton's loved ones have asked for privacy, a family spokesperson told People, which confirmed that the actress died on Saturday in California.

Keaton was a frequent collaborator of director Woody Allen, portraying the titular character in Annie Hall, the charming girlfriend of Allen's comic Alvy Singer. 

The film also garnered Oscars for best picture, best director and best original screenplay, cementing Keaton's place as one of the industry's top actresses and an offbeat style icon as well.

The actress made her mark co-starring in eight Allen movies, from Play It Again, Sam (1972) to Manhattan (1979) and Manhattan Murder Mystery (1993).

In The Godfather films, she played Kay Adams, the girlfriend and eventual wife of Al Pacino's Michael Corleone.

Apart from the Allen cannon, fans adored her portrayal as Steve Martin's wife in 1991 comedy Father of the Bride, as the nervous and doting couple plan an extravagant wedding for their daughter.

Late in her career, Keaton starred in two movies about aging women: Book Club (2018), with its message that love has no age, and Poms (2019), the story of a terminally ill woman who moves to a retirement community to die, but winds up forming a cheerleading squad.

A BAFTA and Golden Globe winner, Keaton scored Oscar nominations three other times for best actress, for Reds, Marvin's Room and Something's Gotta Give.

In 2017, she was honored with a Life Achievement Award by the American Film Institute, which called Keaton "unconventional, iconoclastic and left-of-center."

"I feel so lucky to have spent any time with this marvelous woman, and I'm heartbroken that she is gone," said actress Andie MacDowell, who starred in Unstrung Hero (1995), one of a handful of films Keaton directed.

As the Hollywood sexual harassment scandals detonated in late 2017, cascading from producer Harvey Weinstein to heavyweight actors like Kevin Spacey, old accusations of child sex abuse against Allen by his adoptive daughter Dylan resurfaced.

"Woody Allen is my friend and I continue to believe him," Keaton tweeted in January 2018.

It was a rare encounter with controversy for the beloved actress.

Keaton said she had no "Me Too" complaints despite half a century in the film industry.

"Never. Maybe I just wasn't harassment material," she told AFP in a 2019 interview.

Keaton's infectious, sunny smile lit up the screen across the decades, and she made popular a quirky and liberating fashion sense first seen in Annie Hall that featured oversize hats and the lighthearted use of menswear items.

As for aging, Keaton said in the 2019  interview that life actually got easier.

"I think so, because what have you got to lose? It's like it's the truth. That's what it is. You face it, we talk about it," she said.

Born Diane Hall in Los Angeles on January 5, 1946, Keaton was romantically involved with Allen, Pacino and Warren Beatty, but never married. 

"Most people in the movies get married at some point and then they divorce. But I've never even got married. I am [a] failure," she joked.

Did she regret it? "I don't think about it a lot but I'm aware of the fact that I'm unusual in that regard, and maybe I did miss out on something; but then, nobody can have everything, right?" 

She is survived by her two children, Dexter and Duke.