Nannostomus marilynae (from the Greek nanos, meaning "small", and the Latin stomus, relating to the mouth; marilynae in honor of ichthyologist Marilyn Weitzman), commonly known as Marilyn's or the greenstripe pencilfish, is a freshwater species of fish belonging to the characin family Lebiasinidae. They have been recorded from the Rio Negro and Rio Orinoco regions of Brazil, Venezuela, and Colombia.