Johannes Schlaf (21 June 1862 in Querfurt – 2 February 1941 in Querfurt) was a German playwright, author, and translator and an important exponent of Naturalism. As a translator he was important for exposing the German-speaking world to the works of Walt Whitman, Émile Verhaeren and Émile Zola and is known as a founder of the "Whitman Cult" in Germany. His literary achievements lie foremost in the scenic-dialogue innovations of "sequential naturalism" and in the formalization of literary impressionism. He also contributed to the emergence of the "intimate theater."
Some of his poems have been set to music by composers Alban Berg and Arnold Schoenberg.
He is sometimes mistakenly cited for coining the term "The Third Reich" in relation to Nazism because of his 1906 novel by that name. The Nazi use of the term comes from a 1923 book Das Dritte Reich by Arthur Moeller van den Bruck.