Conservation strategies rely on precise taxonomic delimitation, yet delineating species complexes remains challenging. Despite these difficulties, it is crucial to protect vulnerable complexes. We examined the Poidium complex, comprising five morphologically similar taxa that co-occur in the highland grasslands of northeast Rio Grande do Sul, Brazil. Our goals were to assess whether morphological characters distinguish taxa or groups within the complex, evaluate correlations with environmental and geographic patterns, and inform conservation priorities. Using uni- and multivariate analyses of morphometric and environmental data, we identified three main groups within the complex, each defined by distinctive palea and lemma surface traits. Quantitative spikelet and lemma characters significantly differentiate these groups, and environmental and morphometric variables help characterize subgroups within the clusters. The majority of subgroups are concentrated in Northeast Rio Grande do Sul. Our results support circumscribing Poidium juergensii as having only strigose lemmas and no infraspecific delimitation, as well as treating individuals with glabrous lemmas at the species level, as Poidium tricholepis sp. nov. In addition, we highlight the weak morphological distinction between Poidium calotheca and Poidium ambiguum in sympatric areas. This study underscores the uniqueness of the South Brazilian highland grasslands and the relevance of including species complexes in conservation efforts.