Troubled Manhattan art dealer Helly Nahmad, who was thrown in prison in April for running an illegal gambling ring, is being sued for allegedly hiding a $20 million painting stolen by the Nazis.

Frenchman Phillippe Maestracci, whose grandfather Oscar Stettiner was a Jewish art dealer when he fled Paris before Hitler’s army invaded in 1939, is suing Nahmad and his billionaire art dealer dad David Nahmad in Manhattan Supreme Court.