The New York real estate magnate lagged far behind Clinton in May, raising $3.1 million to her $26 million, according to federal disclosures filed late on Monday.
Trump, who self-funded his primary campaign, more than doubled his previous monthly hauls after holding his first general election fundraiser on May 26.
Trump spent much of the primary shunning donations, telling his supporters not to contribute to his campaign and ridiculing his opponents for accepting donations from special interest groups and wealthy contributors.
The unorthodox candidate, who is still struggling to get full backing from the Republican Party establishment, is betting he can run a race that builds on his low-spending, insurgent primary operation.
"I can go a different route, I could just spend my own money, have a lot of cash and do like I did with the other and just spend money on myself and go happily along and I think I win that way," Trump said on Fox News.
Trump said he raised $12 million during the weekend for the Republican Party.
He has loaned his campaign $46 million since launching last year and did not rule out tapping his personal wealth again.
"I may do it again in the general election," he said on NBC's "Today" show. "But it would be nice to have some help from the party."
With spending that outpaced inflows, his campaign began June with just $1.29 million in cash, well-behind Clinton's $42 million war chest.
The figures underscore the huge money advantage Clinton is hoping to enjoy leading into the Nov. 8 presidential election, one that could allow her a large staff and millions of dollars of television and digital ads in battleground states.
Trump's meager May haul drew a flurry of taunts on Twitter on Tuesday morning, including a #TrumpSoPoor hashtag mocking the billionaire. One post joked he had listed his Mar-a-Lago estate as an Airbnb rental, a platform often used by people trying to earn cash off their homes.
MONEY IS NOT A PROBLEM
Trump advisers said they were not concerned.
"Guess what - Mr. Trump is a billionaire," Tana Goertz, a Trump senior adviser, told CNN on Tuesday. "Money is not a problem."
Trump’s allies have said the cash is now "pouring in" for the general election. For months, the biggest cash injections into his campaign coffers were from his personal bank accounts.
Trump still may have several hurdles to cross before convincing deep-pocketed donors to write the kind of checks that would make him competitive with Clinton’s campaign bank account.
Trump donors, allies and other Republican operatives continue to express concerns about his campaign operation, which has been dogged by internal battles, a threadbare campaign infrastructure of about 30 paid staffers, and a barely existent fundraising apparatus.
On Monday, Trump fired his campaign manager, Corey Lewandowski, who had been overseeing the campaign’s fundraising arm.
The primary Super PAC supporting Trump, Great America PAC, reported raising $1.4 million in May - accounting for most of the $2.5 million the group has raised this year. The PAC had $500,971 cash remaining at the beginning of June.
Clinton’s cash advantage has been fueled in part by the Super PAC supporting her. Priorities USA raised $12 million in the last month, ending May with a $52 million in cash. Three unions, AFT Solidarity, Liuna Building America and International Union of Operating Engineers, each gave $1 million.
The group has largely been tasked with attacking Trump. And so far, they have spent more than $5.7 million this year on television ads alone attacking the Republican.
(Additional reporting by Grant Smith and Michelle Conlin in New York, Eric Walsh in Washington; Editing by Doina Chiacu and Bill Trott)
