Alnus maritima (seaside alder) occurs in wet or flooded soils on the Delmarva Peninsula (southern Delaware and adjacent counties of eastern Maryland, USA) and in small niches in Georgia and Oklahoma. Although some A. maritima exist in coastal estuaries, responses of the species to salt, and whether salt resistance depends on provenance of plant origin, are not known. We first compared salt injury among A. maritima from all provenances irrigated with dilutions of ocean water. Seedlings from the three provenances responded similarly. Salinity up to 3.1 g/kg did not alter stem elongation but did cause increases in the ratio of dry to fresh weight of basipetal leaves, which reflected their necrosis. Photosynthesis and growth after treatment with salt then were compared among A. maritima and four other woody species: Baccharis halimifolia, Ceanothus americanus, Myrica cerifera, and Cornus amomum. Irrigation solutions with up to 12 g/kg salinity had no effect on B. halimifolia and caused no visible damage to M. cerifera. Ceanothus americanus showed foliar necrosis and decreases in photosynthesis at 3, 6, and 12 g/kg. Photosynthesis of A. maritima and C. amomum was sustained after irrigating with solutions at 3 g/kg but was reduced by 6 and 12 g/kg salinity. Our third experiment showed a more pronounced decrease in photosynthesis as salinity increased for A. maritima with roots partially or totally inundated in solutions with salt compared to plants irrigated but not inundated with these solutions. Lastly, symptoms of salt injury among A. maritima planted at locations that differed in salinity along the Broadkill River, an estuary in Delaware, were found to be similar to those we documented among plants treated in a greenhouse. We conclude that necrosis of basipetal leaves is the most apparent symptom of injury caused by salt in the root zones of A. maritima, that low concentrations of salt can lead to damage, and that the rate at which symptoms develop in this riparian species varies with the extent of root-zone inundation with salt water. Alnus maritima was less sensitive than C. americanus to salt but was considerably more sensitive than B. halimifolia and M. cerifera.