Triglochin maritimum L. is a circumboreal aggregate species complex comprised of a number of different cytotypes. Numerical analyses of morphological variation in plants from eastern and central North America and Europe indicate that two species, T. maritimum and T. gaspense Lieth & Löve, can be recognized. Triglochin maritimum is a polymorphic species, of wide distribution that is characterized by densely flowered racemes, erect leaves that are shorter than the scape, and by its long stigmatic papillae. Triglochin gaspense, restricted to coastal marshes of the Maritime Provinces and the State of Maine, is distiguished by its lax racemes, arcuate leaves that are longer than the scape and its short stigmatic papillae. Differences in habitat preference lend further support to this classification.