The new species Muhlenbergia monandra, endemic to Peru, is described and illustrated. The species is distinguished by possessing longer glumes than anthoecium, spikelike inflorescence, and cleistogamous florets with one stamen. The leaf blade anatomy is described, illustrated, and the distinguishing characters are given. Scanning Electron Microscope observations about the epidermis of the lemma and glumes are made. The taxonomic position is discussed and some remarks about the related annual species of South America are included.