Two new subspecies of Alnus maritima, A. maritima subsp. oklahomensis from south-central Oklahoma and A. maritima subsp. georgiensis from northwestern Georgia, are described based on differences in growth habit, leaf and fruit morphology, and geographic isolation. The two new subspecies, along with subsp. maritima found in Maryland and Delaware on the Delmarva Peninsula, formalize the infraspecific classification of three widely disjunct populations of A. maritima, which have undergone considerable divergence since their isolation. Numerical analyses of 23 characters of mature trees from natural sites and 22 characters of seedlings grown in a uniform environment revealed many differences and allowed the determination of relative taxonomic distances between subspecies. Our examination of morphology, growth habit, distribution, and habitat of the three subspecies also rendered a plausible explanation for the peculiar disjunct occurrence of A. maritima.