The Bolsheviks stole his family’s Cezanne masterpiece in 1918, and now it’s on the walls of the Metroplitan Museum of Art, a Frenchman charges in a federal lawsuit.

“Madame Cezanne in the Conservatory,” said to be worth between $50 and $70 million, was one of hundreds of great works of art looted from the collection of Ivan Morozov after the Russian Revolution, his grandson Pierre Konowaloff claims in papers filed in Manhattan federal court late yesterday.

In 1933, Stephen Clark, an American collector purchased the painting along with a Van Gogh and several others through a Russian laundering operation, the lawsuit alleges.

After his death in 1960, the Van Gogh was gifted to the Yale University — which Konowaloff is also suing — and the Cezanne to the Met.

There is ample precedent prohibiting museums from taking works stolen by the Nazis, “but there haven’t been similar cases regarding Bolshevik looted art,” Konowaloff’s lawyer, Allan Gerson told the Post.

Konowaloff did not file the lawsuit sooner because until his father died in 2002, no one had done a full accounting of his family’s stolen works – most of which remain in Russian museums, Gerson said.

Yale University and the Metropolitan Museum of Art both claim to have proper title to the works.

“This lawsuit is totally without merit,” the Met said in a statement. “The Museum intends to defend this lawsuit vigorously.”

Konowaloff and his family want to regain title, or be compensated for the works that rightfully belong to the family, Gerson said.

“They don’t want to deprive the public of a right to see the painting,” he said. “If they have title to it, they promise to put it on public display in a safe place.”