Steinem died "peacefully" at her home in New York City, a statement on her Instagram account confirmed.
"Gloria's near-century on earth were years well-lived, and she continued working for equality until the very end," the statement added.
"Gloria's greatest gift was her ability to listen to others, to make others feel seen and heard.
"Gloria lived true to her independent spirit, always with curiosity and a great sense of humour."
Steinem became one of the most visible symbols and potent voices of the US women's movement and a fierce advocate for feminist causes throughout her life.
With her trademark streaked hair parted down the middle and large aviator glasses, Steinem, a co-founder of Ms. Magazine, was a hugely recognisable presence wherever she went.
Much of her life as an advocate was spent on the road, speaking at college campuses and community centres, at rallies and marches.
"You can't do it on the phone or on the web," she told The Associated Press in a 2015 interview. "You have to be there with all five senses. It should be obvious that people can't empathise with each other unless we're in same room."
She began her career as a journalist, notably one who reported, in 1963, on degrading conditions in Hugh Hefner's Playboy empire, an undercover assignment she came to later regret. She credits a meeting six years later, where women spoke out boldly in favour of abortion rights, with changing her career path.
"That speak-out was the beginning of activism for me," Steinem, who had undergone a secret illegal abortion at age 22, told The AP in an email in 2022, reflecting on the reversal of Roe v Wade.
"Either our control over our own bodies is equal, regardless of sex or race, or we're not living in a democracy."
Steinem founded the National Women's Political Caucus in 1971 with former US Representatives Bella Abzug and Shirley Chisholm and writer Betty Friedan, whose 1963 book The Feminine Mystique ushered in the second wave of feminism.
Among other groups, she also created the Coalition of Labor Union Women, the Women's Media Center, Voters for Choice, and the Ms. Foundation for Women.
In 2013 then-US president Barack Obama awarded Steinem the Presidential Medal of Freedom, the highest US civilian honour.
Steinem continued working into her 80s. She wrote numerous books, including 1986's Marilyn: Norma Jean about Marilyn Monroe, Revolution from Within: A Book of Self-Esteem about personal development, and books of essays.
She narrated and conceived the idea for an Emmy Award-winning documentary about child abuse, Multiple Personalities: The Search for Deadly Memories, for HBO in 1993 and produced the Lifetime film Better Off Dead. She also hosted Woman, a documentary series about violence against women, for the Viceland TV channel.
with AP and PA
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