Dianne Feinstein, a Democrat who has represented California in the Senate since 1992, announced on Monday that she would run for reelection in 2018.
Feinstein made the announcement in a tweet from her campaign's Twitter account.
Feinstein said that she hopes to end gun violence, fix climate change, and improve access to healthcare. "I'm all in!" Feinstein tweeted.
Sen. Kamala Harris, California's other senator, released a statement supporting Feinstein's reelection bid.
The veteran Democrat had dodged questions about whether she would seek a fifth term, leaving her party in the nation's most populous state in limbo about whether to start grooming a possible replacement. The senior senator has faced withering criticism this year from some liberal activists who accuse her of not being tough enough on President Donald Trump.
Gun control has been a top priority for Feinstein since she entered Congress. "Right now we're seeing one president change actions of a - of a president that came before him, and that would happen in this area".
Feinstein's old-school political sensibilities and foreign policy could be at odds with California, where increasingly young and diverse voters have demanded all-out opposition to Trump.
Beyond opposition from the left in the media and with some holding office, Feinstein may face problems with California voters. "We are better off with her leadership and I look forward to continuing to fight together for California in the Senate".
Feinstein also said she had some "interest" from the Congressional Republicans to work on the legislation banning bump stocks, but did not provide any names.
Los Angeles Mayor Eric Garcetti is reportedly hosting a fundraiser for Feinstein.