A judge ruled that Michael Cohen, the president's long-time personal attorney, must reveal he had been retained by Sean Hannity.
Monday's hearing in a New York City court follows an FBI raid this month on Mr Cohen's home and office.
Agents were seeking evidence on various matters, including a $130,000 payment made to porn star Stormy Daniels.
It not is clear why Hannity hired Mr Cohen.
There are unconfirmed reports that the work was linked to an advertiser boycott of Hannity's show by the left-wing Media Matters group.
The conservative host, who is known for passionately defending Mr Trump on his Fox News show, referred to the Cohen hearing at the start of his radio show on Monday.
"I think it's pretty funny," Hannity said. "I'll decide if I'm going to put out a statement here."
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