Peter Julius Hertz (1 June 1874 in Copenhagen – 26 March 1939) was a Danish art historian and museum worker.
He was also interested in contemporary art in Denmark and wrote "Gennembruddet i 70'erne, Betragtninger i Anledning af Raadhusudstillingen og den Hirschsprungske Samling" ("The breakthrough in the 70s, Reflections on the occasion of Raadhusudstillingen and the Hirschsprung Collection" in Art, IV, 1902–03).
As his main interest began to shift more towards contemporary art, he started to advocate for it through making monographs and working on the biographies of L. A.
His first marriage started on 16 December 1899 in Schöneberg town hall in Berlin, with pianist Karen Wellmann (born 24 September 1875) in Køng.
He was portrayed by Herman Vedel in 1901, 1902 and 1903; Fritz Burger in Switzerland; Laurits Andersen Ring; and at least 1,932 other drawings, including some by Ludvig Find (Frederiksborg Museum) and Arne Lofthus.