The song of the Seaside Sparrow (Ammospiza maritima) is a 2-phrase song, with a complex introduction and a terminal buzz, that has previously been shown to exhibit some interindividual and between-population variation. Comparatively little is known about geographic variation in Seaside Sparrow song, or if song structure corresponds to either subspecies or coastal lineages (Atlantic vs. Gulf of Mexico). We collected recordings from all recognized subspecies throughout the species' range to ask whether variation in Seaside Sparrow song corresponds with subspecies or coastal lineage designation. Coastal lineage groups of Seaside Sparrow varied significantly with respect to a number of frequency and temporal characters. Most notably, frequencies of the introductory syllable within the first phrase were higher for the Gulf coast lineage whereas frequency measures for the terminal syllable within the first phrase were lower. Additionally, the number of syllables in the first phrase was greater within the Gulf coast group. A multiple logistic regression analysis using 6 song measures classified coastal lineage groups with 96% accuracy but failed to differentiate 7 subspecies for which we had sufficient data.