Former President Donald J. Trump can proceed with a lawsuit against Mary L. Trump, his estranged niece, over her role as a source for a New York Times investigation into Mr. Trump’s finances, a New York State appeals court said on Thursday.

The ruling, from the Appellate Division of the State Supreme Court, was a victory for Mr. Trump, though it did not address the substance of his claim: that his niece should be held liable for breaching a confidentiality agreement when she provided financial documents to a team of Times journalists.

Those documents became the basis of a series of news articles examining what The Times called Mr. Trump’s history of tax avoidance and “outright fraud.” The series received a Pulitzer Prize for explanatory reporting in 2019.

Mr. Trump sued The Times in 2021 over the articles, accusing the news organization of improperly inducing his niece to provide the documents. Last year, a New York judge dismissed Mr. Trump’s claims against The Times and its journalists; the judge also ordered the former president to pay the newspaper’s legal fees.

In the claims against his niece, Mr. Trump has argued that she broke the terms of a confidentiality agreement that was part of a settlement related to the will of Fred C. Trump, the former president’s father, who died in 1999.

Ms. Trump, who disclosed her role as a source for The Times in a 2020 memoir, maintains that the First Amendment protected her actions.

In a three-page opinion on Thursday, the state appeals court upheld a lower-court ruling that there was “a substantial basis in law” for Mr. Trump’s claim of breach of contract. The opinion also noted several questions that would need to be resolved for Mr. Trump to prevail, including the precise duration of the confidentiality provision signed by Ms. Trump and whether the former president could prove that he suffered damages as a result of the disclosures.

The case now returns to New York State Supreme Court, the state’s trial-level court.

A lawyer for Mr. Trump, Alina Habba, said the former president would continue his lawsuit to ensure that Ms. Trump “is held fully accountable for her blatant and egregious breach of contract.”

A lawyer for Ms. Trump, Anne Champion, said she was “disappointed” by the ruling. “The First Amendment gives us all the right to criticize candidates for office, and Mary has made valuable contributions to the public’s knowledge of the former president with her unique perspective as a family member,” she said in a statement. “She should not be silenced, and we are confident she will be vindicated as the case proceeds.”