Winged bean [Psophocarpus tetragonolobus (L.) DC.] is a vegetable legume crop. The center of origin, diversity and domestication of this crop are not known. In this study, we assessed the genetic diversity and population structure of 457 accessions of winged bean collected from six geographical regions (North, Northeast, East, West, and central, and South) in Thailand using 14 simple sequence repeat (SSR) markers. In total, the SSR markers detected only 55 alleles with an average of 3.9 alleles per locus. Observed heterozygosity was relatively high (0.15) and overall gene diversity was moderate (0.487). Gene diversity, allelic richness and observed heterozygosity in the six regions were comparable, while the estimated out-crossing rate was relatively high (16.4％). STRUCTURE analysis grouped the 457 winged bean accessions into three sub-populations. Neighbor-joining (NJ) analysis grouped all the accessions into two major clusters. Genetic groups identified by both STRUCTURE analysis and NJ analysis were unrelated to geographical origins. Principal coordinate analysis revealed no clear clustering of the winged bean accessions. Although genetic groups were not unrelated to geographical origins, most of the winged bean accessions with long pods (30 cm or higher in length) or having purple seed coats or purple young pods were grouped together. This suggested that the winged beans with long pods or with purple seed or purple young pods may have a single origin. Altogether, these results demonstrated that the genetic diversity of winged bean in Thailand was moderate with high genetic admixture. We argue that the high genetic admixture of the winged bean in Thailand is due to seed migration and relatively high outcrossing rate.