The importance and diversity of maize in the Southwestern United States, and questions about their relationships to environmental and cultural factors, drove the need to characterize and classify known existing landraces. Maize landraces and human cultures have co-evolved, with maize being shaped by diverse environmental and cultural selection pressures. Understanding maize diversity, and relationships between maize landraces can lead to insights into the cultural history of the Southwest and the effect of diverse environmental stress on maize diversity development and utilization. This study examined 134 landraces from the Southwest, 13 landraces from Mexico and 12 Midwestern controls. Fields were located in Farmington, New Mexico and Ames, Iowa. Measurements were taken on phenological, vegetative and reproductive characteristics. Significant differences among accessions, environments and ethnic groups were found. There was also a significant ethnic group by environment interaction. Analysis of the accession by environment interaction, by ethnic group, indicated that several ethnic groups had more variables with significant accession by environment interaction. Principal component and cluster analyses showed a continuum of landraces, with the Pueblo and southern Arizona landraces on the extreme ends and many intermediates. One group of landraces grouped separately, and included Hopi and Tohono O'odham landraces with large ears. This group also included Mexican June, which may reflect the effects of introductions of dent maize into the Southwest. Five primary clusters were identified by the cluster analysis and include a Pueblo cluster, a Pima and Tohono O'odham cluster, an intermediate cluster, a cluster of cornbelt dents and a cluster with large-eared Hopi and Tohono O'odham landraces. The cluster relationships correlate well with language groups and geographic and climatic factors. It is difficult to distinguish the effect of cultural factors, such as geographic isolation and the introduction and spread of Spanish cultural influences, on the relationships seen in the clustering. The racial distinctions found in previous studies of Pima-Papago and Pueblo are confirmed by this study, with minor differences. Relationships with Mexican landraces and Midwestern maize were also examined and can provide insight into maize migration into the Southwest and the effect of introductions of commercial maize on traditional maize landraces.