Alnus maritima is a North America endemic species in the Betulaceae family. This species has a disjunct distribution, having three very widely separate populations; one in Oklahoma, one in Georgia and the other in Maryland and Delaware. Despite some debate over the acceptance of these three subspecies, a recent genetic study has concluded there are three geographically distinct subspecies. Despite its common name of Seaside Alder, this species only occurs in freshwater tidal and non-tidal systems.

Alnus maritima is a multi-stemmed large shrub or small tree. It has smooth, light grey bark and simple elliptical leaves with a singly serrated margin and leathery texture. Vivid yellow, floral catkins give rise to brown cone-like fruits which release the water-dispersed seeds. This species is unusual as in blooms in the autumn, making it the only member of its genus native to North America to do so, all others bloom in spring. This autumn-blooming phenology is a characteristic only shared with two old-world Alnus species, both endemic to southeast Asia, making them the nearest extant relatives to A. maritima.

Climate change poses a significant threat to the whole species, but especially to A. maritima subsp. maritima which is endemic to Maryland and Delaware. This subspecies is threatened by rising sea levels as salt water intrusion into current fresh water tidal systems will lead to local extirpation of this population. Other threats to this species include grazing and low genetic diversity. These threats are predicted to cause decline in area and quality of habitat and the number of sub-populations. Furthermore, due to the distinct genetic identity of each subspecies, reduced genetic diversity within each subspecies and the lack of gene flow between subspecies, it is vitally important to protect all populations so to conserve the genetic diversity and long term viability of the species as a whole.