Sida uniaristata, a new species of Sida sect. Sidae L. from the Atlantic Forest of Brazil, is described. Morphologically, the new species is characterized by having 1-awnedmericarps, whereas in the remaining species of the genus the mericarps are always 2-awned, muticous, or rostrate. Comments about the geographic distribution, conservation status, and morphological affinities of S. uniaristata, as well as images and a key to the new species and similar taxa within Sida sect. Sidae, are provided. [ABSTRACT FROM AUTHOR]