Relationships between the flora and vegetation of northern Europe and the northeastern United States have been long noted but little studied. Here, quantitative sampling of high, low, and mid-marsh zones was carried out in 20 marshes along a gradient from non-tidal freshwater via tidal freshwater and brackish to salt marshes in each of two large river systems, the Elbe in Europe (Germany) and the Connecticut in North America (southern New England, USA). The composition of each zone of each marsh type in the two river systems is described. The marsh floras are compared, and possible reasons for differences in vegetation composition, non-native species occurrences, species richness patterns, and the origin of marsh floras are discussed. Twenty-two species occurred in both river systems, nearly 10% of the total marsh flora. Shared species were found in all four marsh types and occasionally in more than one marsh type. Introduced taxa are not prominent in either river system except for Spartina anglica C. E. Hubb. in the salt marshes and Acorus calamus L. in the non-tidal freshwater marshes of the Elbe and Typha angustifolia L. and Phragmites australis (Cav.) Trin. ex Steud. in the brackish marshes along the Connecticut. Asteraceae and Poaceae were the largest (most species rich) families in the marshes of both rivers. These descriptions and comparisons of the marsh flora and vegetation of a European and an American river should serve as benchmarks for future comparisons and assessments of changes in times of rapid environmental and associated ecological change.