Feinstein, who has held the seat since 1992, has faced pressure to resign given that she is the oldest member of Congress at 89 years old. If she were re-elected, she would be 97 at the end of her six-year term.
Porter, 49, is a rising progressive star in the Democratic Party.
The former law professor has become a social media sensation for her whiteboards and rapid-fire grilling of powerful bank and drug company executives over issues like compensation.
She has championed inflation-based caps for drug prices and making it unlawful to excessively hike gasoline prices.
She was a top fundraiser in the House in November's midterm elections, raising over $25 million to win her competitive district for the third time.
"California needs a warrior in the Senate - to stand up to special interests, fight the dangerous imbalance in our economy, and hold so-called leaders like (top Senate Republican) Mitch McConnell accountable for rigging our democracy," she said in a video posted to Twitter. She made no reference to Feinstein.
Feinstein is a trailblazer in U.S. politics: the first woman to serve as mayor of San Francisco, first woman to serve on the Senate Judiciary Committee and now the longest-serving female senator.
"Everyone is of course welcome to throw their hat in the ring, and I will make an announcement concerning my plans for 2024 at the appropriate time," Feinstein said in response to Porter's announcement.
(Reporting by Moira Warburton; Editing by Scott Malone and Lisa Shumaker)
By Moira Warburton
