The son of a German wartime art dealer, who had more than 1400 paintings in his Munich flat, has told a German news magazine they are his and he wants them returned.
German prosecutors believe that almost half the paintings were looted by the Nazis.
But Cornelius Gurlitt, 80, who inherited them, told Der Spiegel magazine the paintings were "acquired legally".
Investigations are continuing to establish who the original owners might be.
The paintings were discovered in Mr Gurlitt's apartment in March last year during a routine tax inspection.
They include works by Marc Chagall, Pablo Picasso, Henri Matisse, Otto Dix and Max Liebermann. They are being held at an undisclosed location.
The BBC reports Jewish families and museums say the paintings were taken from them by the Nazis and are calling for them to be returned.