During the last ten years, water blooms of Aphanizomenon have been observed in aquatic environments of the subtropical region of Argentina. These blooms have been observed to present taxonomic characteristics similar to those of A. platensis, a species originally described by Seckt in the Delta of Paraná River, a region of temperate climate, in 1921. Water blooms of Aphanizomenon have been observed in aquatic environments of Corrientes Province, a region of Argentina with subtropical climate, during the last ten years. Considering these aspects, we analysed the morphological variation of A. platensis in two water bodies located in the northwest of Corrientes province (27° 29′ S; 58° 45′ W). Our description includes characteristics of the akinetes, which were not originally found by Seckt. The revision of this taxon is important because the species of Aphanizomenon are potentially toxic, invasive, and bloom formers. Our study confirms the existence of this morphospecies almost 90 years after it was first described. We further describe the whole species and extend its distribution to the subtropical region, in environments related to the hydrographic net of the Rio de la Plata Basin. Taking into account the diacritic characters analyzed, we may conclude that A. platensis is a polymorphic species with highly variable cell dimensions, probably due to variations in the environmental conditions.