The US Supreme Court has ruled that President Trump's financial records can be examined by prosecutors in New York.
In a related case, the court ruled that this information did not have to be shared with Congress.
Mr Trump is the first president since Richard Nixon in the 1970s not to have made his tax returns public.
His lawyers had argued that he enjoys total immunity while in office and that Congress had no valid justification to seek the records.
Two Democrat-controlled House of Representatives committees and New York District Attorney Cyrus Vance - also a Democrat - had demanded several years' worth of Mr Trump's taxes in order to determine whether current conflict-of-interest laws on a US president were tough enough.
Mr Trump, a Republican, denies wrongdoing and has called the investigation into his tax affairs a "witch hunt".
"The Supreme Court sends case back to Lower Court, arguments to continue. This is all a political prosecution," he wrote in a series of tweets following the court rulings.
The two cases regarding Congressional committees were closely watched, as they could have had implications on how far US lawmakers can scrutinise the activities of a sitting president.
In the case regarding the request from the New York prosecutors, the Supreme Court ruled by a majority of seven to two that the president did not have absolute immunity from criminal investigation.
"Two hundred years ago, a great jurist of our Court established that no citizen, not even the President, is categorically above the common duty to produce evidence when called upon in a criminal proceeding," the court said.
"We reaffirm that principle today."