Salvinia molesta, commonly known as giant salvinia, is a free-floating aquatic fern native to southeastern Brazil. It is invasive in many parts of the world, especially southern Africa, where it is commonly known as Kariba weed due to its infestation of Lake Kariba, a large artificial lake situated between Zambia and Zimbabwe. The fern is notable for its long and broad fronds and bristly, waterproof surfaces. The fronds are produced in pairs, with a third modified root-like frond that hangs in the water.

S. molesta has been accidentally introduced to several lakes throughout the world. Since 2019, it has been included in the European Union's list of invasive alien species of Union concern. The species cannot be legally imported, cultivated, transported, commercialized, planted, or intentionally released into the environment in the European Union. In the United States, it has been introduced into Caddo Lake in Texas, where it has caused significant eutrophication.