"While I concede this election, I do not concede the fight that fuelled this campaign," she told supporters at her alma mater Howard University, a historically black college, on Wednesday.
Harris pledged to continue fighting for women's rights and against gun violence and to "fight for the dignity that all people deserve".
She said she had called President-elect Trump, congratulated him on his triumph and promised to engage in a peaceful transfer of power.
Harris addressed a crowd that included former House Speaker Nancy Pelosi, aides in President Joe Biden's White House and thousands of fans who listened to a soundtrack that included Beyonce's Run the World (Girls) and Tye Tribbett's We Gon' Be Alright.
Her running mate, Minnesota Governor Tim Walz, joined the crowd.
Crowds gathered at Howard University in Washington for the concession speech. (AP PHOTO)
Harris rose to the top of the Democratic ticket in July after Biden stepped aside and brought new-found enthusiasm and cash to the Democratic ticket, but struggled to overcome voters' concerns about the economy and immigration.
She was handed a resounding loss, with Trump winning a greater share of votes across most of the nation compared with his performance in 2020 and Democrats failing to secure key battleground states that decide elections.
Thousands had gathered at Howard University, Harris' alma mater, on Tuesday night for what they hoped would be a historic victory for the first woman to become president.
They came back on Wednesday to show their support after her loss.
