Franz Carl Mertens (3 April 1764 – 19 June 1831) was a German botanist who was a native of Bielefeld.

Mertens studied theology and languages at the University of Halle, and after graduation taught classes at Bremen Polytechnic College.

In his spare time he studied botany, and through a mutual friend met German botanist Albrecht Wilhelm Roth (1757–1834).

He also performed illustrative work on Volume 3 of Roth's Catalecta botanica.

With Friedrich Heinrich Wilhelm Frölich and Johannes Nicolaus von Suhr he issued the exsiccata-like series Hydrophytorum tam indigenorum quam exoticorum collectio, Sammlung von einheimischen und auswärtigen Hydrophyten (1829).