De Niro was called to the stand Friday as a witness in the trial of Leigh Morse, the director of an art gallery whose owner is accused of being part of a scheme to chisel artists out of their share of the sale of their works.
The owner of the Salander-O'Reilly gallery, Lawrence Salander, was convicted last year of stealing money and artwork from the artists he represented.
The New York Times said Saturday that De Niro testified in State Supreme Court that he became suspicious of Salander during a trip to Europe in which they each took separate private planes.
De Niro later heard from an artist that Salander had sold off some of his paintings to another gallery.
"I felt something was going to happen down the line," De Niro testified. "It didn't bode well for me hearing that."