Miscanthus sinensis Anderss. is a perennial grass being considered for bioenergy production in the United States (USA) and elsewhere. Because M. sinensis is already naturalized in the USA, it is important to examine its potential to spread with further introduction. Introduced populations may harbor substantial genetic diversity or enable genetic admixture, which may enhance adaptation to novel environments. Here we compare genetic diversity in introduced US populations with native Japanese populations to understand the potential for invasiveness through genetic means in M. sinensis. Using 11 microsatellite markers, we determined genetic variation and structure in populations within and across the introduced and native ranges. We also correlated genetic diversity with population size in each range. Genetic diversity was substantial and similar in all populations, with only allelic richness (A, Ap) significantly lower in the USA than in Japan. Japanese populations were slightly more differentiated (θ = 0.098) than those in the USA (θ = 0.078) although both exhibited significant genetic admixture and were composed of a number of different genetic groups (K = 3 or 7, depending on the STRUCTURE model). Genetic diversity (He) was positively correlated with population size, but only in the USA The lack of a genetic bottleneck and the consequent maintenance of genetic diversity and admixture in US populations suggest the potential for further spread of introduced populations, assuming such diversity is important for adaptation to novel environments. Potential invasiveness should be considered by breeders of new Miscanthus varieties for bioenergy and ornamental uses.