California Governor Gavin Newsom has chosen Democratic strategist Laphonza Butler to fill the state's vacant US Senate seat after the death of Senator Dianne Feinstein.
Mr Newsom had pledged to appoint a black woman if Ms Feinstein's seat became open. The long-serving US senator died last week at the age of 90.
“I have no doubt she will carry the baton left by Senator Feinstein, continue to break glass ceilings, and fight for all Californians in Washington,” the governor said in a statement.
Ms Butler, who is gay, will become the only black woman to serve in the Senate.
She is the president of Emily's List, a political organisation that helps to elect Democratic women who support women's rights. She will step down from her position on the organisation, Mr Newsom said.
“Laphonza has spent her entire career fighting for women and girls and has been a fierce advocate for working people,” he said.
Ms Butler was also an adviser to Vice President Kamala Harris during the 2020 presidential election.
Ms Butler currently lives in Maryland, according to records from the Federal Election Commission. A spokesperson for Mr Newsom said Ms Butler owns a home in California and is expected to re-register to vote in the state before she is sworn into office.
Her appointment will also fill a crucial seat for Democrats, who hold a narrow 51-49 majority in the Senate. Once sworn in, Democrats will have more leeway to pass legislation and confirm appointments that Republicans oppose.