Feinstein's office did not immediately respond to a request for comment on the news, first reported by the Punchbowl news outlet.
Feinstein was a Washington trail-blazer who among other accomplishments became the first woman to head the influential Senate Intelligence Committee.
During almost 31 years in Senate she amassed a moderate-to-liberal record, sometimes drawing scorn from the left.
Feinstein joined the Senate in 1992 after winning a special election and was re-elected five times including in 2018, along the way becoming the longest-serving woman senator ever.
Feinstein's political career was shaped by guns.
She became San Francisco's mayor in 1978 upon the assassinations of Mayor George Moscone and Supervisor Harvey Milk. Feinstein was president of the San Francisco County Board of Supervisors when Moscone and Milk were gunned down by a former supervisor. After hearing the gunshots, she rushed to Milk's office. While searching for his pulse, her finger found a bullet hole.
Feinstein said the horror of that experience never left her and she went on to author the federal ban on military-style assault weapons that lasted from 1994 until its 2004 expiration.
FILE - San Francisco Mayor Dianne Feinstein carries a candle as she leads an estimated 15,000 marchers also carrying candles during a march in memory of slain Mayor George Moscone and Supervisor Harvey Milk in San Francisco, Nov. 28, 1979.
This is a gun-happy nation, and everybody can have their gun," Feinstein said after a May 2021 mass shooting in her home state as she lamented years of congressional failure to pass new gun control laws to guard against "the killing of innocents."
Feinstein led a renewed effort for tougher gun laws including a fresh ban on assault-style weapons after a 2012 massacre of 20 children and six adults at a Connecticut elementary school. The legislation encountered furious opposition from Republicans and gun rights advocates and failed in the Senate.
Health issues slowed Feinstein late in her career, as she became the oldest serving senator. She announced in February 2023 that she would not seek re-election next year and was absent from Congress for three months this year, ending in May after suffering from shingles and complications including encephalitis.
As Intelligence Committee chair, Feinstein overcame resistance from national security officials and Republican lawmakers in 2014 as her panel released a report detailing the CIA's secret overseas detention and interrogation of foreign terrorism suspects following the Sept. 11, 2001, attacks on the United States by al-Qaeda militants.
"The CIA's actions are a stain on our values and our history," Feinstein said, defending the release of a report that revealed CIA use of "coercive interrogation techniques in some cases amounting to torture" on at least 119 detainees.
"History will judge us," Feinstein added, "by our commitment to a just society governed by law and the willingness to face an ugly truth and say, 'Never again.'"
Despite CIA claims that the practices had saved lives, the report concluded that the interrogation methods had played no role in disrupting any terrorism plots, capturing any militant leaders or finding al-Qaeda chief Osama Bin Laden, who American forces killed in Pakistan in 2011.
The late Arizona Senator John McCain, tortured as a prisoner of war in Vietnam, praised Feinstein's release of the report and said, "Most of all, I know the use of torture compromises what most distinguishes us from our enemies."
Protecting America
Feinstein defended U.S. surveillance programs exposed in 2013 by National Security Agency contractor Edward Snowden, a leak she called "an act of treason."
"It's called protecting America," Feinstein said of the NSA electronic surveillance of telephone data and Internet communications that critics called a vast government over-reach.
During Republican George W. Bush's presidency, Feinstein backed the 2002 Iraq war resolution but later voiced regret. She supported Bush's Patriot Act to help track terrorism suspects, but criticized him for authorizing spying on U.S. residents without court approval.
At times, critics on the left felt she was not liberal enough or insufficiently antagonistic toward Republicans. For example, some liberal activists called on her to resign in 2020 after she hugged Republican Senator Lindsey Graham following a Judiciary Committee confirmation hearing for Republican President Donald Trump's conservative Supreme Court nominee Amy Coney Barrett.
She castigated Trump in 2021 after his supporters attacked the Capitol in a failed bid to overturn Democrat Joe Biden's 2020 election victory. She said Trump was "responsible for this madness" for inciting people to violence with false claims of widespread election fraud.
Born on June 22, 1933, Feinstein grew up in San Francisco and graduated from Stanford University. She was elected in 1969 to the San Francisco County Board of Supervisors and became its president in 1978. She became San Francisco's first woman mayor and was elected to two full terms.
FILE - U.S. Senator Diane Feinstein (D-CA) is pushed in a wheelchair on Capitol Hill in Washington, U.S., Sept. 20, 2023.
She ran for governor in 1990, winning the Democratic primary but losing to Republican Pete Wilson in the general election. Feinstein then ran in 1992 for the Senate seat that Wilson had previously held, easily winning the seat. She was the first woman elected to the Senate from California and became the state’s longest-serving senator.
California’s governor, Democrat Gavin Newsom, will name a temporary replacement for Feinstein’s seat, until an election can be held.
Feinstein's first marriage ended in divorce. She then married Bertram Feinstein, a surgeon. After his death, she married Richard Blum, an investment banker, in 1980. He died in 2022.