"Here's what we know: In the first quarter of this campaign, supporters gave more than USD 45 million," Clinton wrote in a tweet.
Final figures will be released once her campaign files its fundraising report in July.
Signaling successful grassroots efforts to raise modest amounts across a broad platform of supporters, her campaign said that 91 per cent of the donations from April through June were USD 100 or less.
In today's fundraising email, campaign manager Robby Mook described it as "a new record for this early in a campaign."
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It surpasses the USD 41.9 million raised by President Barack Obama in 2011 at the start of his re-election campaign, according to The Washington Post.
The donations, capped at USD 2,700 per individual, are to be used in the primary race for the party's nomination.
Team Clinton had reportedly set a goal of raising USD 100 million in 2015.
The new haul leaves Clinton well ahead of her rivals for the Democratic nomination, including liberal Senator Bernie Sanders, former Maryland governor Martin O'Malley and Rhode Island ex-governor and senator Lincoln Chafee.
Clinton's first-out-of-the-gate money announcement signals her team's confidence in voter support as she gears up for a long-term campaign battle.