President-elect Donald Trump’s charitable foundation engaged in self-dealing in 2015 and prior years, the foundation said in an Internal Revenue Service filing.

That self-dealing, which typically triggers additional taxes for anyone who got improper benefits, resulted from payments to “disqualified persons,” or foundation insiders, according to the document. The document, which was the foundation’s tax return for 2015, doesn’t provide more details. Representatives for Mr. Trump didn’t respond to requests for comment Tuesday.

The tax form asks if any income or assets were transferred to a disqualified person, and the foundation checked “yes.” The signed, undated form was posted on Guidestar, a website that compiles nonprofits’ tax forms. Guidestar said it was submitted by the law firm representing the foundation and that it might not be the exact final version that gets submitted to the government.

“They admit at this point that there are problems with their prior returns,” said Phil Hackney, a Louisiana State University law professor who has been examining the returns.

The tax code requires nonprofits to release their tax returns publicly. As a result, the foundation’s filings give voters one of the few peeks they have had into Mr. Trump’s dealings with the IRS. He was the first major-party presidential candidate in 40 years to refuse to release his tax returns.