This study was conducted to assess the genetic diversity and population structure of 70 amaranth accessions collected from South and Southeast Asia using 14 simple sequence repeat (SSR) markers. In total, 67 alleles were detected, with an average of 4.79 per locus. Rare alleles comprised a large portion (46.3%) of the detected alleles, and 29 unique alleles associated with rice accessions were also discovered. The mean major allele frequency (MAF), genetic diversity (GD) and polymorphic information content (PIC) of the 14 SSR loci were 0.77, 0.36, and 0.34, respectively. A model-based structural analysis revealed the presence of three subpopulations. The genetic relationships revealed by the neighbor-joining tree method were fairly consistent with the structure-based membership assignments for most of the accessions. All 70 accessions showed a clear relationship to each cluster without any admixtures. We observed a relatively low extent of genetic exchange within or among amaranth species from South and Southeast Asia. The genetic diversity results could be used to identify amaranth germplasms and so facilitate their use for crop improvement.