Obama praised 51-year-old Harris as a "lifelong courtroom prosecutor" who has fought international gangs, oil companies and the big banks responsible for the mortgage crisis.
"Kamala Harris fights for us. That's why I'm so proud to endorse her for United States Senator," the president said in a statement released by the Harris campaign and the Democratic National Committee.
Harris she faces fellow Democrat Loretta Sanchez, a 10- term congresswoman in the November polls.
"And if you send her to the Senate, she'll be a fearless fighter for the people of California - all the people of California - every single day," Obama said.
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For Sanchez, the Orange County congresswoman, the endorsements are a stinging rebuke and another political obstacle to the many she must overcome by the November election, the Los Angles Times reported.
Harris, who was born in Oakland, California, is the daughter of an Indian mother who emigrated from Chennai in 1960 and a Jamaican American father.
The president's nod caps a string of major endorsements for Harris, the candidate of choice among the Democratic Party's power barons and some of the left's most influential interest groups. It also sends a clear signal to Democratic donors, many of whom have stayed on the sidelines this election, the paper said.
Harris said she was honoured to receive the support of the president and vice president, calling them "incredible leaders for our nation."
Harris has more than a 3-to-1 edge over Sanchez in fund- raising and easily topped a crowded field of Senate candidates in the June primary. The attorney general also held a 15- percentage-point lead over Sanchez in the latest Field poll, the paper said.
The two Democrats will face off in the November election, setting the stage for the highest-profile contest between two members of the same party since California adopted a top-two primary election system.
Obama reciprocated by helping to launch Harris into the national spotlight when he gave her a speaking role at the 2012 Democratic National Convention in North Carolina.