US Senator Dianne Feinstein said on Tuesday that she would not seek re-election at the end of her term in 2024, ending a groundbreaking political career spanning six decades.
It also clears the path for a hotly contested race among California Democrats for her seat.
The announcement was widely expected as Ms Feinstein, who turns 90 in June, is the oldest member of Congress.
"I’ve served with more US senators than just about anyone. I can honestly say that Dianne Feinstein is one of the very best," President Joe Biden said in a statement.
"I look forward to continuing to work with her as she serves out her term."
In recent years, questions have been raised about her cognitive ability and memory, although she has defended her effectiveness in representing a state that is home to nearly 40 million people.
Ms Feinstein said she intended to remain in Congress until the end of her term.
“Even with a divided Congress, we can still pass bills that will improve lives," she said. "Each of us was sent here to solve problems.
"That’s what I’ve done for the last 30 years, and that’s what I plan to do for the next two years."
Ms Feinstein is a trailblazer in US politics. She was the first female mayor of San Francisco, first woman to serve on the Senate judiciary committee and is now the longest-serving female senator.
"Often the only woman in the room, she was determined to lift America up, and through her intellect, empathy, character and drive, to make this country everything it could be," Mr Biden said.
She was expected to serve this year as president pro tempore, the ceremonial head of the Senate and third in line to the presidency, a position typically given to a senior senator of the majority party.
But she declined to seek election for the position, months after Senate majority leader Chuck Schumer, a fellow Democrat, would not say whether he had confidence in her ability to serve.
Instead, Senator Patty Murray, 72, was installed in the job.
Last year, media outlets reported that Ms Feinstein's memory was rapidly deteriorating.
California reliably votes Democratic and the state is home to many up-and-coming politicians who will be looking at her seat.
Several Democrats had already announced or hinted at runs even before her announcement, including Representatives Katie Porter and Adam Schiff.
California will hold its primary elections on March 5, 2024.
News agencies contributed to this report
