Alnus maritima, the seaside alder or brook alder, is a species of shrub or small tree in the family Betulaceae. Alnus maritima is endemic to the United States, and is found naturally in three disjunct populations in Oklahoma, Georgia, and in Maryland and Delaware on the Delmarva Peninsula.

Alnus maritima is the only autumn-blooming member of the genus Alnus native to North America. All other North American alders bloom in the spring. The autumn-blooming phenology is a characteristic that Alnus maritima shares with two old-world Alnus species, Alnus nitida and Alnus nepalensis, which are endemic to southeast Asia. This profound similarity in their timing of anthesis has led to their classification as the only three members in the subgenus Clethropsis.

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