We describe Echinodorus nenufar, a new species from ephemeral aquatic environments in the dry Chaco region of northern Argentina and western Paraguay. This species is readily distinguished by its unique floral coloration—pale yellow to creamy white petals with a basal yellow blotch—and its floating leaf blades, which are broadly cordate to suborbicular, evoking the morphology of waterlilies (Nymphaeaceae). The species exhibits exceptional phenotypic plasticity, producing linear, ovate, or rounded leaves depending on its developmental stage and hydrological context. It completes its life cycle in temporary ponds and seasonal wetlands, where multiple life stages may coexist simultaneously. Molecular phylogenetic analyses based on nuclear (ITS, LEAFY) and plastid (rpl22–rps19) markers confirm the distinctiveness of E. nenufar within the E. grandiflorus complex. We also introduce a novel protocol for preserving delicate aquatic plant structures for herbarium curation. Due to its aesthetic appeal, adaptability, and ease of cultivation, E. nenufar might become a valuable ornamental species.