Trump has previously challenged a summons from New York State Attorney General Letitia James, demanding that he, daughter Ivanka Trump and eldest son Donald Trump Jr. testify.
The verdict, which was hammered out on Thursday by the state judge Arthur Engoron, means that all three must appear for testimony under oath.
"On sale"
The announcement was preceded by a two-hour hearing in which the former president's lawyers tried to block the verdict.
One of them, Alina Habba, accused, among other things, the Minister of Justice of conducting a "vendetta against Donald Trump and his family".
- If she wants a testimony under oath from my client, he has the right to immunity.
Either he gets immunity for what he says, or he says nothing at all, said Ronald Fischetti, another of Trump's defenders, during the hearing.
Letitia James considers herself to have evidence that the Trump Organization has submitted false valuations of real estate in order to be able to take out a loan and seize tax benefits.
The decision is expected to be appealed.