Donald J. Trump, the presumptive Republican presidential nominee, said on Wednesday that he had raised $26 million for his campaign in June, a substantial improvement from his paltry May total but one that still leaves him far behind his rival, Hillary Clinton.

The donations came from 400,000 supporters, including a $3.8 million contribution made by Mr. Trump to his own campaign. The candidate also raised $25 million for Trump Victory, a joint venture with the Republican National Committee that is being used to help elect Republicans across the country, putting his overall haul for the month at $51 million.

Mr. Trump, a self-proclaimed billionaire, has so far contributed $55 million to his campaign, and he has said that he will not seek to be reimbursed for those funds.

Mr. Trump stepped up his fund-raising efforts last month after raising just $1.3 million in May. Mrs. Clinton raised $40 million for her campaign in June and another $28 million for the Democratic National Committee and state parties.