A strategist who runs an organisation devoted to electing Democratic women who support abortion rights is to replace US senator Dianne Feinstein, who died last week at the age of 90.
California Governor Gavin Newsom appointed 44-year-old Laphonza Butler to the role.
She will be the only black female in the Senate, and the first LGBTQ+ person to represent California in the chamber.
Ms Feinstein, a trailblazer for female politicians, was the oldest senator.
Under Californian law, the governor can appoint a senator until the next state-wide election.
Ms Butler is the first black woman to lead EMILY's List. Before she started the role in 2021, she was a senior campaign adviser to US Vice-President Kamala Harris during her 2020 presidential bid.
Ms Butler has also worked as director of public policy at Airbnb and as a union leader.
Last month, prior to Ms Feinstein's death, Mr Newsom pledged to appoint a black woman to serve the remainder of Ms Feinstein's term - until elections take place in November 2024 - should the post become vacant. However, he said he hoped that it was a decision he would not have to make.
Ms Feinstein had previously announced plans to retire at the end of next year, but resisted growing calls for her to step down. Several prominent Democrats, including Congress members Adam Schiff and Katie Porter, have previously said they will run for her Senate seat.
In a statement after her death, US President Joe Biden said that Ms Feinstein "made history in so many ways, and our country will benefit from her legacy for generations".
Earlier this year, she was absent from Capitol Hill for nearly three months following a case of shingles. She took on lighter duties upon her return and moved around the US Capitol using a wheelchair. She sometimes seemed confused during interviews, and in committee hearings or floor votes.
Ms Feinstein was well known as a vocal advocate for gun control measures, and an ardent supporter of the assault weapons ban signed into law by President Bill Clinton in 1994.
As a senator, Ms Feinstein was the first woman to chair the powerful Senate Intelligence Committee, where she led a years-long review of the CIA's controversial interrogation programme of foreign terrorists in the wake of the 9/11 attacks in 2001. The review ultimately led to legislation barring the use of "enhanced interrogation techniques" such as waterboarding on terrorism suspects.
