The Mongolian steppe is among the largest undeveloped grasslands in the world. However, degradation of this vegetation has recently been recognized as a serious national problem. Detailed systematization of the vegetation based on species composition is required to mitigate losses in this unique habitat. We aimed to compile information on steppe vegetation in western Mongolia, where there is still a dearth of data. Vegetation data were collected in 2010 and 2011 using phytosociological procedures around the periphery of the Great Lakes basin and the southern Khangai Mountains. Plant communities were then differentiated on the basis of differential species identified from the abundance data for each stand. Four main phytosociological plant communities were identified: Agropyron cristatum—Artemisia frigida, Echinops gmelinii—Artemisia rutifolia, Anabasis brevifolia—Stipa glareosa, and Hordeum turkestanicum—Artemisia caespitosa communities. The Agropyron cristatum—Artemisia frigida community was broadly distributed in western Mongolia; the species composition of this community was representative of steppe communities in western sectors of the country. Other communities were members of the class Stipetea glareosae-gobicae, which integrates the desert and desert steppe communities. Some diagnostically important species of steppe communities in the central sector were absent from the steppe communities distributed in western Mongolia. Moreover, characteristic species of desert steppe communities, such as Stipa glareosa, Stipa gobica, Allium polyrhizum, and Gypsophila desertorum, occurred in the steppe vegetation.