A New York judge has ruled that Donald Trump and his children must answer questions under oath.
The Trump Organization has been accused of using fraudulent or misleading asset valuations to get tax breaks.
Within 21 days, Mr Trump must testify.
He is expected to appeal the decision.
He has the right to remain silent in a deposition.
The entire system is corrupt, according to a statement from the Trump Organization.
Mr Trump, his son Donald Trump Jr, and daughter Ivanka Trump must comply with legal orders that prosecutors issued in December, according to the New York Supreme Court Judge.
New York Attorney General Letitia James was given a "clear right" to question the former president and two of his children under oath after uncovering "copious evidence" of possible financial fraud.
Mr Trump has called the investigation a witch-hunt by Ms James, a Democrat.
Whether Donald Trump will answer questions under oath about his business practices is an open question.
An appeal is likely to be forthcoming. Eric Trump declined to answer questions under similar circumstances in 2020 if that is unsuccessful.
That is what the former president's lawyers recommend. Having a client sit for a multi-hour grilling by New York lawyers is not their idea of sound legal strategy.
It was a rough day for the former president, but it was far from the end of the line. The civil case and criminal investigation in Manhattan are not going away soon. It is not a good look for a man who likes to boast of his unparalleled business prowess to have a judge say that there is evidence of fraud.
The investigation aims to prove that Mr Trump inflated the value of his assets in order to get a loan. The fraud is said to have taken place before he took office.
Attorneys for Mr Trump tried to prevent Ms James from questioning the former president and his children.
Ms James said that justice had prevailed.
The civil case is unrelated to the criminal investigation of the Trump Organization.
Attorneys for Mr Trump argued in court that testifying in the civil case would allow the state to circumvent a law that bars prosecutors from calling someone to testify before a criminal grand jury unless they are granted immunity.
Mr Trump's defence attorney said that he was entitled to immunity if Ms James wanted to give sworn testimony.
He gets immunity for what he says or nothing.