We surveyed plant diversity in the tidal fresh, brackish and salt marshes of the 10-km long Saco River estuary in southern Maine. We also investigated possible relationships between shoreline development in the estuary, nitrogen levels in marsh sediments, and marsh plant species composition. Plant species cover was determined in 1 m² quadrats along transects in sixteen marsh study sites, and land cover was mapped in a 100-m buffer around each site. Nitrogen content (total nitrogen, nitrate and ammonium) of marsh sediments was measured at ten sites. Marsh porewater salinities were determined to range from 0.2±0.1 to 18.6±2.9 ppt (mean±1 SE), with species diversity the greatest in marshes where salinity was less than 8 ppt. We identified 72 vascular plant species in the estuary's marshes, including ten that are listed as threatened or of special concern in the state of Maine. Ordination results showed that the extent of development along the shoreline correlated with the variation in plant species abundance at tidal marsh sites. The proportion of high intensity development also correlated negatively (r_s(16) =-0.571, p=0.021) with plant diversity as measured by the Shannon Wiener index. Nitrogen levels in marsh sediments did not explain these results. The results of this study establish a baseline for future studies of tidal marsh plant communities in the Saco River estuary, and show that the extent of shoreline development may influence tidal marsh plant diversity, even across a salinity gradient.