Nannostomus mortenthaleri, commonly known as the coral red pencilfish, is a freshwater species of fish belonging to the characin family Lebiasinidae. It is one of the most colourful of the genus, being suffused with a bright coral red colouration over its entire body and fins, in striking contrast to its horizontal jet black stripes. It was originally described as a subspecies of Nannostomus marginatus, but it is now recognised as a species in its own right. Mature males have a thickened anal fin. Maximum length is 29mm.
