One of the most extensive grassland ecosystems in the Neotropics is located in the southeastern South America region. Here grasslands once dominated the Pampas but these are now mostly reduced to a mosaic of patches with different land-uses, largely croplands and pasturelands. Native Cortaderia selloana grasslands are widely distributed in the eastern Pampas region but relatively little is known of the bird assemblage inhabiting this habitat. We studied this bird assemblage addressing the overall richness and presence of species of conservation concern, seasonal variation in species composition, breeding phenology and the importance of C. selloana habitats for birds. Species richness was high: 54 species belonging to 22 families, including six species of conservation concern: three classified as vulnerable to extinction (dot-winged crake Porzana spiloptera, black-and-white monjita Xolmis dominicanus and pampas meadowlark Sturnella defilippii), and three near threatened (greater rhea Rhea americana, bay-capped wren-spinetail Spartonoica maluroides and Hudson’s canastero Asthenes hudsoni). The highest species richness was observed in spring and summer, with peaks in spring and marked drops in autumn. The insectivore guild was the most numerous throughout the year. We identified 21 nesting species, four of which were of conservation concern, with a peak of reproductive activity during November. Owing to the high species richness and of the large number of threatened species, C. selloana grasslands should be considered of outstanding conservation priority in the Pampas region.