There has been great confusion between Spermacoce glabra, S. riparia, and S. tenuior since their original descriptions. These species can be distinguished mostly by their seed features. Clustering and principal coordinate analyses (PCoA) were used to scrutinize the formation of hierarchical non-overlapping groups strictly according to the degree of morphological similarity. Thirteen characters were scored for 26 herbaria specimens. The taxonomic identity of these three species is discussed. Redefined descriptions and illustrations are provided, especially for fruit, seed, and pollen characters. A map and a discussion of the updated geographical distribution of these species are included. A table summarizing the different taxonomic concepts in contemporary floristic studies and an identification key to all American Spermacoce species are also provided. The new combination Spermacoce affinis (DC.) R.M. Salas is presented here.