Harris is the first candidate to officially join the contest and is expected to be a serious contender for the first Senate seat to open up in California in more than two decades.
In an email titled "Join me on a new journey," Harris announced her plan to supporters and immediately began raising money for a campaign that could cost tens of millions of dollars, depending on who else joins the field.
"Your support has been crucial to me every step of the way, and I'm asking you to help me build a grassroots campaign that reaches every community of California," Harris wrote.
She repeatedly referred to herself as a fighter, an echo of the persona that has long defined Boxer, who announced last week that she would not seek reelection in 2016.
"I want to be a voice for Californians on these issues and others that impact our state in the U.S. Senate," Harris said. "I will be a fighter for the next generation on the critical issues facing our country."
The news of her bid was greeted by encouraging statements from Democratic groups and women's organizations.
"We're excited to watch her candidacy — it's critical Senator Boxer's seat stays in the hands of a champion for women and families," said Marcy Stech, spokeswoman for EMILY's List, a group that works to elect pro-choice Democratic women to office.
Although the organization offered no official endorsement, it emailed its 3 million members about Harris' announcement and has the ability to spend heavily in support of her bid.
But others expressed concern about her commitment to the environment and consumers.
"Boxer has been a leading champion for consumer safety and economic protections on a whole range of issues, including auto safety. We have reason to doubt that Kamala Harris has as much fight in her on consumer issues," said Rosemary Shahan, president of Consumers for Auto Reliability and Safety.
Harris' entry into the race puts pressure on others who are considering a run. Within hours of her announcement, two potential candidates put out statements laying out the foundations of their potential bids.
"Washington needs to be shaken up, and we need climate champions who will fight for the next generation," billionaire environmental activist Tom Steyer wrote in the Huffington Post. " … I will decide soon based on what I think is the best way to continue the hard work we have already started together to prevent climate disaster and preserve American prosperity."
Steyer, who is also being urged by some supporters to consider a run for governor in 2018, is expected to announce his intentions within days.
"Californians deserve a strong voice in Washington, and I have never been afraid to speak up, which is why I am seriously considering running for the United States Senate in 2016," Sanchez said in a statement, touting her work on
Former Los Angeles Mayor