The scientific community has long assumed that plant breeding activities decrease genetic diversity in crop species. To determine the influence of plant breeding on peanut, this study was designed to assess allelic diversity changes among peanut (Arachis hypogaea L.) cultivars of the runner market type using simple sequence repeat (SSR) markers. All runner-type cultivars released to date were included with the exception of ten cultivars released in the 2000s. Thirtyfour SSR primer pairs amplified a total of 154 alleles. The results indicated that (i) at the gene level, allelic diversity has increased significantly through decades of breeding, (ii) at the population level, genetic diversity was at its lowest during the pre-1980s time period and gradually increased in each subsequent decade, and (iii) most of the observed SSR variation occurred within, rather than among time periods. A principal coordinate analysis (PCO) clearly demonstrated increases in the variation present in each subsequent breeding decade, reaching its maximum in the 2000s. Therefore, it appears that runner-type peanut breeders have been successful at developing improved peanut cultivars while increasing levels of diversity in the last three decades of breeding. In addition, genetic relationships among cultivars reported in this study might be of use for peanut breeders when selecting parents for establishment of breeding populations. © Crop Science Society of America.