The aides added that the 400,000 donations came from about 250,000 individual donors. The average donation was $33.51 and 99% of donations were under $250.
The Sanders' campaign will not release its full Federal Election Commission report on Thursday. Aides said that will come later this month.
Sanders' fundraising was done primarily online and at his rallies, which have lately drawn large crowds, including one with nearly 10,000 people
Wednesday night in Madison, Wisconsin, the largest crowd for any presidential candidate's event so far this cycle. During the rally, campaign volunteers handed out envelopes that asked people to donate to the campaign.
The Independent Vermont senator openly abhors fundraising, and has so far appeared at less than a handful of in-person fundraisers.
Instead, the Sanders campaign has focused on digital donations and hired Revolution Messaging, a consultancy made up of former Obama staffers, to handle its digital outreach.
"We have (raised money) very, very differently than other campaigns," Sanders told CNN earlier this week. "But I believe we will be able to raise, as will be indicated in this reporting period and in the future, enough money to run a winning campaign."
Sanders' campaign has drawn comparison to another Vermonter who ran an insurgent race for President: Former Gov. Howard Dean, whose campaign revolutionized the way campaign money was raised by tapping into online donations. In the second quarter of 2003, Dean's campaign raised $7.6 million, primarily online.
Sanders' top aides hope to raise between $40 and $50 million this year, a number substantially less than the $100 million Hillary Clinton's campaign is aiming for.
The reason for the difference is simple: Sanders is running a smaller, comparatively stripped-down campaign that won't spend money as quickly as Clinton.
Clinton's campaign announced an impressive haul of at least $45 million
in her first quarter as a candidate, smashing the previous record set by President Barack Obama in 2012.
Clinton, however, spent considerably more time fundraising than Sanders. In total, Clinton appeared in front of 58 fundraisers that raised at least $23 million, according to a CNN analysis.
For the Sanders team, Thursday's fundraising announcement was as much about the need for money to sustain the campaign as it was about providing their longshot campaign with credibility in the eyes of voters, journalists and other Democrats.
"I think it is very important," Tad Devine, Sanders' top campaign strategist, said days before the campaign released their fundraising numbers. "One of the things we have tried to do is to demonstrate ... that this is a real campaign."
Devine added, "This first quarter is going to be a benchmark in that process of are we credible and can we actually raise that kind of money."
Sanders and his aides have tried to turn the fact they will be outraised by Clinton and most Republicans into a selling point. In fundraising emails earlier this week, Jeff Weaver, Sanders' campaign manager, asked donors for $3 apiece, even though the campaign "long suspected that other candidates are going to raise a lot more money than we will. That's what happens when you have Wall Street and the billionaire class in your corner."
Only Sanders and Clinton have detailed their quarterly fundraising totals so far. Campaigns for former Maryland Gov. Martin O'Malley and former Rhode Island Gov. Lincoln Chafee said this week that they will likely not announce their numbers this week. Campaigns have until July 15 to file their fundraising totals with the FEC.