Feinstein, who was the longest-serving female senator in U.S. history, passed away last week at the age of 90.
Butler is the president of the pro-abortion group EMILY's List and previously worked at Airbnb as a director of public policy. She will become the third Black female senator to serve in Congress.
"As we mourn the enormous loss of Sen. Feinstein, the very freedoms she fought for – reproductive freedom, equal protection, and safety from gun violence – have never been under greater assault. Laphonza has spent her entire career fighting for women and girls and has been a fierce advocate for working people," Newsom said in a statement on X.
California will hold two elections for the Senate seat in November. The first election will be a special one to finish the remaining months of Feinstein's term until January 2025. The second election will decide who will hold the seat for the full six-year term.