Franz Mertens (March 20, 1840 - March 5, 1927) was a German mathematician. He was born in Schroda near Posen, Kingdom of Prussia (now Sroda, Poland) and died in Vienna, Austria.

The Mertens function is the sum function for the Möbius function, in the theory of arithmetic functions. The Mertens conjecture concerning its growth, conjecturing it bounded by x^1/2, which would have implied the Riemann hypothesis, is now known to be false (Odlyzko and te Riele, 1985). The Meissel-Mertens constant is analogous to the Euler-Mascheroni constant, but the harmonic series sum in its definition is only over the primes rather than over all integers.

See also: Mertens' theorems.