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The floodplain of the rivers Danube, March/Morava and Thaya/Dyje in eastern Austria and western Slovakia harbours a great diversity of meadows, reed swamps and sedge-bed communities. However, the grasslands along the Danube have not been adequately addressed by any study up to now, and a transnational revision of the alluvial grasslands is completely lacking. In this third part of a series focusing on the syntaxonomy of the Pannonian grasslands of Austria, we present a detailed classification of the grassland and marsh vegetation of the Danube and March-Thaya floodplain. We compiled all available relevés from the study area belonging to the classes Phragmito-Magnocaricetea, Molinio-Arrhenatheretea and Festuco-Brometea. In total, our data set comprised 2119 relevés, of which 355 were from Slovakia. We conducted a TWINSPAN classification and, based on a provisional syntaxonomic interpretation of the clusters, assigned all relevés to classes, orders, alliances and associations using the total cover of the diagnostic species in each relevé as the assignment criterion. We identified 42 associations and five provisional communities belonging to 14 alliances. Our revision includes substantial changes to previous overviews, in particular regarding the alluvial grasslands of the March-Thaya floodplain. We merge Lathyro palustris-Gratioletum, Gratiolo-Caricetum suzae, Cnidio-Violetum pumilae, Serratulo-Plantaginetum altissimae and "Silaetum pratensis" into only two associations (Gratiolo-Caricetum suzae and Cnidio-Violetum pumilae, alliance Deschampsion), which are differentiated along the moisture gradient. The Ophioglosso-Caricetum tomentosae is revealed as a geographical vicariant of the Cnidio-Violetum pumilae, replacing the latter along the Danube. The Agropyro-Alopecuretum pratensis is newly reported for Austria. The mesic Festuca rupicola grasslands along the March/Morava (previously named "Serratulo-Festucetum commutatae") are included in the Colchico-Festucetum rupicolae (Cirsio-Brachypodion). Ranunculo bulbosi-Arrhenatheretum, Pastinaco-Arrhenatheretum, Festuco rupicolae-Brometum and Polygalo-Brachypodietum (the latter in a new subass. selaginelletosum helveticae) are confirmed as widespread grassland types in the Danube floodplain, and the Teucrio botryos-Andropogonetum (Festucion valesiacae) is split into two subassociations. Moreover, we report four grassland types dominated by Elymus repens and Calamagrostis epigejos, provisionally treated as rankless communities, which have been neglected by all previous authors. [ABSTRACT FROM AUTHOR]