Robert Friedrich Wilhelm Mertens (1 December 1894 – 23 August 1975) was a German herpetologist. The Robert Mertens' day gecko is a species named after him, as is Mertens' water monitor, and he also postulated Mertensian mimicry.
Born in Saint Petersburg, Russia, Mertens moved to Germany and attended the University of Leipzig, earning his PhD in zoology there in 1916.
After working at the Senckenberg Museum in Frankfurt for several years, Mertens became a lecturer at the University of Frankfurt in 1932. In his capacity as museum curator, and also in the various posts he held at the University, he was able to travel extensively and study lizards (particularly varanids, lacertids, and geckos) in many different locations, including Indonesia, Namibia and El Salvador.
He authored several books on zoology, including La Vie des Amphibiens et Reptiles in 1959. An English translation, The World of Amphibians and Reptiles, was published in 1960. Mertens described at least 59 reptile species and numerous amphibians. In addition, at least eight reptile species have been named after Mertens, e.g., Varanus mertensi, a monitor.
Mertens died after being bitten while feeding his pet savanna twigsnake, Thelotornis capensis.