Joseph Gregor (* 26 October 1888 Czernowitz – 12 October 1960 Vienna) was an Austrian writer, theatre historian and librettist.
Joseph Gregor was born in Czernowitz. He studied musicology and philosophy at Vienna University, graduating in 1911. He worked under Max Reinhardt as assistant director and from 1912-14 as a lecturer in music at the Franz-Josephs-University of Chernivtsi. He was employed at the Austrian National Library in Vienna in 1918. There he founded the Theatre Collection in 1922, in which he included film after 1929. He also taught from 1932–38 and 1943-45 at the Max-Reinhardt-Seminar. In 1953 he retired from the service of the National Library.
Gregor's role in the era of National Socialism is disputed controversially: Gregor incorporated many libraries of politically persecuted intellectuals into the Austrian National Library. Some say, he did so in order to save these libraries, others say, Gregor profited from the policital persecution. An example is the autograph collection of Stefan Zweig in 1937, were Oskar Pausch sees a promise by Gregor to protect the collection after the regime change in 1938, whereas others see the role of Gregor more critical; the same is valid for the acquisition of the theater collection of Helene Richter (1861-1942) for the Austrian National Library. In 1940 Gregor took over the library of Heinrich Schnitzler, with whom he was friends.
His grave is located in the columbarium of the Feuerhalle, Simmering (Vienna). His son Čestmír Gregor became a noted composer.
A year after the seizure of power by the Nazis in Germany, Jewish librettist Stefan Zweig fled to London, leaving Richard Strauss to look for a new librettist. Originally recommended by Zweig, Joseph Gregor wrote three librettos for Richard Strauss: Friedenstag (1938), Daphne (1938) and Die Liebe der Danae (1944), as well as contributing to the texts of Capriccio (1942) and the posthumous school opera Des Esels Schatten. Never completely convinced by Gregor as a librettist, Strauss rejected his drafts for three other works: Celestina, Semiramis and Die Rache der Aphrodite.