Johann Friedrich Pfaff (sometimes spelled Friederich; born Stuttgart, 22 December 1765, died Halle, 21 April 1825) was a German mathematician. He was described as one of Germany's most eminent mathematicians during the 19th century. He studied integral calculus, and is noted for his work on partial differential equations of the first order (Pfaffian systems as they are now called) which became part of the theory of differential forms; and as Carl Friedrich Gauss's formal research supervisor.

Pfaff was a student of Abraham Kästner, and became the precursor of the German school of mathematical thinking, which under Gauss and his followers largely determined the lines on which mathematics developed during the nineteenth century. He knew Gauss well, when they both lived together in Helmstedt in 1798. August Möbius was later his student.