Here we describe and illustrate Dizygostemon riparius (Plantaginaceae, Gratioleae) as a new species from the state of Maranhão, northeastern Brazil. We present morphological and micromorphological descriptions and comparisons, along with illustrations, photographs, and information on its taxonomy, habitat, geographic distribution and conservation status. Dizygostemon riparius differs from D. floribundus, the only other accepted species of the genus, by having the stems sparsely to densely tomentose, the sepals as long as or slightly longer than the corolla tube, the posterior sepal to 1.5 times as wide as any of the other four, the ovary glandular villous, and the leaves larger. The new species is assessed as Endangered (EN) according to IUCN categories and criteria. Additionally, we provide diagnostic features that help to differentiate Dizygostemon from the similar Brazilian genera Achetaria, Stemodia and Tetraulacium.

Citation: Scatigna A. V., Brandão C. M., Colletta G. D., Teles R. de M., Cavalcante K. S. B., Souza V. C. & Simões A. O. 2019: Dizygostemon riparius (Plantaginaceae, Gratioleae), a new species from Maranhão, northeastern Brazil. – Willdenowia 49: 177–186. doi:  https://doi.org/10.3372/wi.49.49206

Version of record first published online on 27 June 2019 ahead of inclusion in August 2019 issue.