A taxonomic revision of the grass genus Koeleria from the study of 3,255 specimens ranging from the western Mediterranean basin and Macaronesia is presented. We include descriptions and nomenclatural synonyms for all taxa inhabiting these regions. Detailed morphometric descriptions, illustrations, distribution maps, an identification key, and habitat data are given for each taxon. We recognize 10 species of Koeleria in this area, grouped into four sections. Four new combinations are provided for Koeleria, K. sect. Imbricatae (Domin) Quintanar & Castrov., K. sect. Reticulatae (Domin) Quintanar & Castrov., K. pyramidata subsp. arenaria (Dumort.) Quintanar & Castrov., K. pyramidata subsp. schroeteriana (Domin) Quintanar & Castrov., and one new name, K. rodriguez-graciae Quintanar & Castrov. Thirty-three names are lectotypified.