The buckwheat Eriogonum corymbosum is widely distributed throughout the southwestern United States, forming a complex of eight varieties. Eriogonum corymbosum var. nilesii is a primarily yellow-flowered taxon reported only from Clark County, Nevada. A previous genetic study by our research group found that variety nilesii is genetically distinct from other E. corymbosum varieties, based on a limited number of populations. Here, we assess genetic variation in 14 newly sampled yellow-flowered populations from southern Nevada, southern Utah, and northern Arizona, and compare them to genetic variation in six populations of previously determined E. corymbosum varieties. Of the new populations, we identified four as variety nilesii, four as variety aureum, three as variety glutinosum, two as apparent hybrids involving varieties aureum and nilesii, and one as a more distantly related admixture. Our results extend the range and area of E. corymbosum var. nilesii considerably. However, this extended range is confined to the Mojave Desert region of southern Nevada, and the number of known populations remains limited.