Helanthium tenellum, the pygmy chain sword, is a species of plants in the Alismataceae.

It is native to the eastern United States (from Texas to Florida, north to Michigan and Massachusetts), southern Mexico (Chiapas, Veracruz), West Indies (Cuba, Jamaica, Hispaniola), Central America, South America (from Guyana to Argentina)[1][2] The leaves of H. tenellum are narrowly elliptical or lanceolate.

During ripening, the sepals enlarge and reach a length of up to 6 mm and fully cover the aggregate fruit.

The achenes are compressed, and arched in the apical part; they are (0.9) – 1.1 – (1.3) mm long, and black, with three lateral ribs.

[7] Helanthium tenellum is a common aquarium plant with a large variation in shapes and sizes kept by people for their aesthetic appeal.