It is widely accepted that in coastal wetlands a negative relationship exists between plant species richness (number of species) and salinity. However, the distribution of species richness across estuarine salinity gradients has not been closely examined. We hypothesized that plant species richness in coastal marshes (i.e., wetlands dominated by herbaceous plants) is highest within the fresh-brackish transition zone (oligohaline salinity regime). The basis for this hypothesis was that salinity fluctuations between fresh and brackish salinities (i.e., 0–7 ppt) might promote coexistence of freshwater and salt-tolerant brackish marsh species, resulting in a peak in richness in the transition zone. We conducted an observational study across the fresh (< 0.5 ppt) to mesohaline (5–18 ppt) salinity gradients of the Nanticoke and Patuxent Rivers of Chesapeake Bay to describe the distribution of plant species richness across each estuarine gradient. A series of 1,000-m² plots (with nested subplots) was established along 50-km sections of each river. Our results do not conclusively support our hypothesis of a transition zone peak in richness for either river gradient. However, richness in transition zone oligohaline marshes was as high as or higher than in tidal freshwater marshes, resulting in a distinctly non-linear pattern of plant species richness along the relatively undisturbed Nanticoke River. In contrast, the more urbanized Patuxent River gradient displayed a linear decrease in plant species richness with increasing distance downstream across the estuary. Hence the non-linear pattern of plant species richness observed along the Nanticoke River may be the typical pattern in relatively undisturbed estuaries.