Sanders reported an adjusted gross income of nearly $561,293 and paid $145,840, a 26% effective rate, in 2018, the documents show. In 2016 and 2017, Sanders reported earning $1.06 million and $1.13 million in adjusted gross income and paid at a 35% and 30% effective rate, respectively.
The effective rates and income both represent substantial increases from 2014, which was the last year of tax returns publicly released by the candidate, when he earned $205,271 and paid a 13.4% effective rate. Sanders has said in interviews that his recent increase in income is due to the success of his books: 2016's "Our Revolution," which came out after Donald Trump took the presidency and earned a spot on the Times' bestseller list and 2018's "Where We Go from Here: Two Years in the Resistance."
Sanders reported earning over $850,000 as an author in 2017 and nearly $800,000 as an author in 2016.
His millionaire status, paired with decades of avid criticism of the millionaire and billionaire class, sets him up for scrutiny in a crowded field of Democrats running for the 2020 nomination.
"These tax returns show that our family has been fortunate. I am very grateful for that, as I grew up in a family that lived paycheck to paycheck and I know the stress of economic insecurity. That is why I strive every day to ensure every American has the basic necessities of life, including a livable wage, decent housing, health care and retirement security." Sanders said in a statement.
"I consider paying more in taxes as my income rose to be both an obligation and an investment in our country. I will continue to fight to make our tax system more progressive so that our country has the resources to guarantee the American Dream to all people," he continued.
Though Sanders is not the only millionaire in the group — Sen. Kamala Harris of California reported an income of $1.9 million between her and her husband, who is an attorney — he is most well-known for having made his career by railing against the economic inequalities caused by America’s richest, a system he criticizes for leaving the bottom 99% with less money than the top 1%.
In a recent op-ed, he outlined his now-familiar promise to take on the “powerful special interests” stemming from “the greed of Wall Street, the power of gigantic multinational corporations and the influence of the global billionaire class.”
In 2016, a major attraction for his strong cohort of young voters was his promise of free tuition at all public colleges and universities, paid for with a new tax on Wall Street trades.
Sen. Elizabeth Warren of Massachusetts, who has also made anti-corruption a key focus of her campaign, reported the next highest income out of the six Democrats who have released at least a decade of returns, showing an adjusted gross income between her and her husband of over $846,000. In past years, her adjusted gross income has been over $1 million and neared $1.55 million in 2014, after her first book was released.
Warren released 10 years of her tax returns last year and followed with an 11th year of released returns last week.
Other Democratic hopefuls Sen. Kirsten Gillibrand of New York and Washington Gov. Jay Inslee have also released returns dating back to 2007. Sen. Amy Klobuchar of Minnesota released 12 years of tax returns through 2017 earlier this month.
Steven Rosenthal, a senior fellow at the Urban-Brookings Tax Policy Center, said the 2020 Democratic primary candidates have set a new standard for financial transparency for future White House bids.
“We’re seeing more returns, and sooner, than ever before,” Rosenthal told ABC News. “I think we’ll see every serious candidate on the Democratic side of the ledger release their tax returns.”
The wait for Sanders’ tax returns lasted well over a month following his promise in a CNN town hall in February that they would be coming “soon.”
Sanders released a portion of his 2014 tax returns last presidential election cycle, followed by his full 2014 tax returns in the face of pressure. He promised to release more returns if he was the party's nominee. But his efforts at transparency paled in comparison to his opponent for the Democratic nomination Hillary Clinton, who released eight years of her tax returns.
"Bernie Sanders has been filing detailed financial disclosures for almost 30 years, and he is proud to voluntarily make these tax returns available many months before the election," Sanders' campaign manager Faiz Shakir said in a statement after the returns were released.
"Senator Sanders believes it is a privilege to live in the United States and he believes it is patriotic to pay the taxes that support our country. As a strong proponent of transparency, the senator hopes President Trump and all Democratic primary candidates will disclose their tax returns," Shakir said.
Up against criticism that, like Trump, Sanders was not being transparent, the senator said he was going to release them on Tax Day and urged the president to do the same.
In an interview with the New York Times, he said his tax returns would be “more boring” because he’s not a billionaire with investments in Saudi Arabia or “wherever he has investments, all over the world.”
Sanders also defended his new millionaire status.
“I wrote a best-selling book,” he said in the interview. “If you write a best-selling book, you can be a millionaire, too.”
It’s tradition for presidential candidates, and most candidates for federal office, to release their tax returns. Trump has repeatedly said he will not because he’s under audit, though Democrats have not retreated from the issue.
Democrats in the House renewed efforts again in early April to force the president’s hand in delivering at least six years of personal and business returns.
On the Senate side, Warren recently introduced a bill that would force candidates to release their tax returns by law.
ABC News' Benjamin Siegel contributed to this report.