Friedrich Moritz "Fritz" Hartogs (20 May 1874 – 18 August 1943) was a German-Jewish mathematician, known for his work on set theory and foundational results on several complex variables.

Hartogs was the son of the merchant Gustav Hartogs and his wife Elise Feist and grew up in Frankfurt am Main.

He studied at the Königliche Technische Hochschule Hannover, at the Technische Hochschule Charlottenburg, at the University of Berlin, and at the Ludwig Maximilian University of Munich, graduating with a doctorate in 1903 (supervised by Alfred Pringsheim).

As a Jew, he suffered greatly under the Nazi regime: he was fired in 1935, was mistreated and briefly interned in Dachau concentration camp in 1938, and eventually committed suicide in 1943.

In set theory, he contributed to the theory of well-orders and proved what is also known as Hartogs's theorem: for every set x there is a well-ordered set that cannot be injectively embedded in x.