Johann Gustav Fischer (1 March 1819, Hamburg – 27 January 1889) was a German herpetologist.
He served as an instructor at the Johanneum in Hamburg, and was associated with the city's Naturhistorisches Museum, working extensively with its herpetological and ichthyological collections.
He was the binomial author of numerous herpetological species, and has several species named in his honor:
"Fischer's cat snake" (Toxicodryas pulverulenta), 1856.
"Fischer's chameleon" (Kinyongia fischeri ), described by Anton Reichenow in 1887.
"Fischer's dwarf gecko" (Lygodactylus fischeri ), described by George Albert Boulenger in 1890.
"Fischer’s snail-eating snake" (Sibon fischeri ), described by George Albert Boulenger in 1894.
"Fischer's thick-toed gecko" (Pachydactylus laevigatus), 1888.In the field of ichthyology, he described the genera Sclerocottus (Cottidae) and Gymnelichthys (Zoarcidae).