Hillary Clinton and her husband Bill, the former president, reported $10.6 million in income for 2015. They paid $3.6 million in federal income tax, according to the document, which was posted on her campaign website.
Over the course of their careers, the Clintons have published all of their tax returns since 1977. Democrats repeatedly point to this as evidence of transparency on the part of the power couple.
Clinton's running mate Tim Kaine also released his returns for the past 10 years on Friday.
"Donald Trump is hiding behind fake excuses and backtracking on his previous promises to release his tax returns," Clinton campaign communications director Jennifer Palmieri said. "What is he trying to hide?"
Trump has so far declined to release his, arguing that his tax returns for the past several years are being audited.
But Democrats hint that the brash billionaire real estate mogul may be trying to hide that he pays little to no tax, makes less money than he claims or gives a negligible amount to charity.
Trump, 70, presented the Federal Election Commission with a mandatory personal financial disclosure form in May and says this is enough.
That document gave only an estimate of Trump's assets, liabilities and income.
Trump has said he is worth more than $10 billion, but no one has been able to confirm this independently. Forbes magazine estimates Trump is worth less than half that - $4.5 billion.
In a report published Friday, The New York Times said Trump had likely benefited from tax breaks available to real estate developers in past years, as in 1978 and 1979, when he paid no federal income tax at all, according to documents he made public in 1981 when applying for a casino license in New Jersey.
Media under fire
The Clintons' main sources of income were Bill Clinton's paid speeches, to the tune of $5.2 million, and a payment to Hillary Clinton from the publisher of her last book, Simon & Schuster, for $3 million.
Hillary Clinton, 68, gave dozens of paid speeches in 2013, 2014 and 2015 but gave up on that before launching her White House run in April 2015.
The Clintons also donated $1 million to the non-profit foundation that bears their name.
The Republican and Democratic candidates in the last nine presidential elections -- since Ronald Reagan in 1980 -- have released tax returns at least for the previous year, according to PolitiFact.
Most released their returns for several years, such as Bob Dole in 1996. He did so for the previous 29 years, according to The Washington Post.
In 2012, Mitt Romney released two years' worth of tax returns.
Trump turned a deaf ear to calls from his critics for his tax returns at a campaign rally Friday in Pennsylvania, a key swing state where he is behind Clinton in the polls.
His sinking poll numbers are worrying some Republicans, but party boss Reince Priebus weighed in Friday to show his support.
"Don't believe the garbage you read -- Donald Trump, the Republican Party, all of you, we are going to put him in the White House and save this country together," Priebus said.
Trump -- launching a broadside at Clinton over her husband's signing of the North American Free Trade Agreement (NAFTA) in 1993, which Trump says cost American jobs -- also took aim at one of his favorite targets: the media.
Incredulous that the media did not see sarcasm in his repeated statements about President Barack Obama and Clinton being the "founders" of the Islamic State group, Trump earned cheers when he said that journalists "are the lowest form of life."
"They are the lowest form of humanity. Not all of them. They have about 25 percent that are pretty good, actually," he added.
