Democrat Kevin de Leon, leader of the California Senate, said Sunday he will challenge.
The Los Angeles Democrat announced via email that he's ready to run on his progressive record fighting climate change and pushing for immigration reform, women's rights and quality education.
His announcement did not mention the 84-year-old Feinstein by name.
De Leon, 50, said he wants to join the U.S. Senate to fight the policies of President Donald Trump.
"He disregards our voices. Demonizes our diversity. Attacks our civil rights, our clean air, our health access and our public safety," the campaign email said about the president.
De Leon will make a formal public announcement in Los Angeles on Wednesday.
He was elected to the state Senate in 2010 and became its president pro tem in 2014. De Leon previously served in the California Assembly.
Feinstein, who has been in the Senate since 1992, is its ranking Democrat on the Judiciary Committee and has never faced a serious electoral challenge from within her party.
She said last week that that she'll run for her fifth full term and quickly was endorsed by fellow Democrats including California's other U.S. senator, Kamala Harris, and Los Angeles Mayor Eric Garcetti.
California Senator Dianne Feinstein announced her intentions to seek re-election in a tweet, telling followers, "I'm all in!"
"I am running for reelection to the Senate. Lots more to do: ending gun violence, combating climate change, access to healthcare," the Democrat wrote. Feinstein has focused, often in futility, on gun control issues and immigration. She is also a senior member of the powerful Appropriations Committee, a post that has enabled her to tend to California's needs, and the intelligence panel.
She is an environmentalist but worked with House Majority Leader Kevin McCarthy, a California Republican, to broker an agreement on divisive Central Valley water issues that was opposed by former Democratic Sen. Barbara Boxer.
Feinstein, who had a pacemaker implanted in January, is the oldest senator in a chamber where it's not uncommon for people to serve into their 80s.
The Democrat will be 85-years-old as she goes before voters next year, and would be 91 by the time her next term ends, if she's elected to a fifth full term.