Although the Hermit Thrush (Catharus guttatus) exhibits distinctive morphological and genetic differences across its breeding range (Alvarado et al. 2014), comparatively little is known about geographic variation in song structure and whether song variation corresponds with those morphological and genetic differences. To address this, we collected recordings from throughout the species' breeding range and assessed frequency and temporal characteristics of song structure. Eastern and western birds differed dramatically with respect to song structure, most notably with respect to introductory note frequencies: eastern birds exhibited a larger range of introductory note frequencies than western birds. They also differed with respect to temporal characteristics: western birds sang longer introductory notes, but shorter post-introductory portions, than eastern birds. Numerous differences in structure were also detected within each lineage. For example, within the western lineage, introductory note frequencies were much higher in the Western Lowland group that occupies the Pacific coast compared to the inland Western Mountain group. There were also some differences seen within those groups; for example, in the western Canadian subspecies C. g. euborius recordings from high altitudes (>1900 m in the Canadian Rockies) were structurally distinct from those recorded at lower altitudes. Highlighting the distinctive geographic differences in song structure, a discriminant function analysis using song structure characteristics classified recordings into their geographic groups with 97.0% accuracy. This array of geographic song differences, taking place at multiple levels, suggests that Hermit Thrush song has been shaped by a variety of forces, ranging from large-scale geological events to factors such as habitat differences and cultural drift that operate on a smaller geographic scale (i.e. within subspecies).