Tim Kaine, Clinton's running mate, will also release 10 years of his returns, added the source close to Clinton.
Trump, however, has declined to release any of his tax returns, citing the fact that he is under IRS audit. The government agency, however, has said that an audit does not restrict Trump from releasing the returns.
It is standard for presidential candidates to release their tax returns during their campaign, providing voters and journalists a glimpse into how they make their money, where they donate it and what tax rate they pay.
Clinton released eight years of personal income tax returns in 2015. With that release, Clinton's aides said 38 years of Clinton tax returns have been available to the press over the course of her career, dating back to 1977.
Hillary and Bill Clinton paid an effective federal tax rate of 35.7 percent and a combined federal, state, and local effective rate of 45.8 percent last year. Clinton's 2015 tax rate, the source said, is expected to be consistent with her past returns.
Paul Manafort, Trump's campaign manager, said in May that he doubts he will release the taxes because they are "incredibly complicated."
The fact that Trump won't release his taxes has raised questions about the actual size of his fortune and some of his businesses connections.
Even other mega-rich businessmen have faulted Trump for declining to release his returns.
"You will learn a whole lot more about Donald Trump if he produces his income tax returns," Warren Buffett, the billionaire head of Berkshire Hathaway, said at a Clinton rally in Omaha, Nebraska earlier this month.
Buffet went on to make Trump an offer "I hope he can't refuse."
"I would be delighted to meet him any place, any time, between now and the election," he said. "I will bring my tax return, he can bring his tax return, no one is going to arrest us, there are no rules against showing your tax returns, and just let people ask us questions about the items that are one there."
Clinton, too, has used Trump's refusal to release his tax returns against him, suggesting to crowds that they may show he is not as wealthy as he says he is.
Clinton, in an economic speech Thursday outside Detroit, will hit Trump for not releasing his taxes and suggest that the returns cover up how much the businessman's tax proposal would actually help people like him, an aide said Wednesday.