Selection during varietal improvement has been shown to reduce genetic diversity in several different crop species. A reduction in genetic diversity can be detrimental to future breeding efforts and increase susceptibility to biotic stresses. The purpose of this study was to analyze changes in levels of allelic diversity at the gene and population levels in 40 zoysiagrass (Zoysia spp. Willd.) cultivars released between 1910 and 2016 using simple sequence repeat (SSR) markers. Fifty-six SSR primer pairs were used to determine whether allelic diversity has changed among cultivars released over a century of modern plant breeding. While no significant differences were observed for total numbers of alleles or genetic similarity values between the initial and most recent time periods, our results demonstrate that genetic diversity among zoysiagrass cultivars has fluctuated over time. Allelic diversity declined during the 1990s and then recovered during 2000s. STRUCTURE analysis revealed five subpopulations with varying levels of admixture and that, for the most part, good representation of these subpopulations has been maintained over the different decades of release. These findings illustrate that plant breeding has not resulted in a substantial decline in zoysiagrass genetic diversity, but also that the potential exists for increasing diversity through future germplasm collections and the inclusion of less-used Zoysia species to create new combinations of alleles.