ABSTRACT The grass genus Orinus , endemic to the Qinghai‐Tibet Plateau (QTP), has long been taxonomically controversial. Recent integrative studies combining morphological and DNA barcoding proposed revising the genus from six to three species; however, this hypothesis lacks validation from genome‐wide data, and the evolutionary history of the putative species remains poorly resolved. Here, we conducted whole‐genome resequencing of 126 individuals from 40 populations representing three putative species of Orinus — Orinus thoroldii , Orinus kokonorica , and Orinus intermedius —and performed population genomic and demographic analyzes. Genome‐wide evidence robustly supports the three‐species delimitation and reveals pronounced genetic differentiation among lineages. We further detected both historical and recent hybridization between O. kokonorica and O. intermedius , indicating incomplete reproductive isolation. Demographic inference suggests that divergence among lineages occurred during Quaternary climatic oscillations, when repeated climate fluctuations, together with the complex topography of the QTP, likely promoted lineage divergence while allowing persistent and asymmetric gene flow, particularly during early stages of divergence. Across species pairs, we consistently identified genomic islands of divergence characterized by elevated absolute nucleotide divergence ( D xy ). Genes located within these regions show signatures of positive selection and are enriched in functions related to environmental adaptation, suggesting a role of natural selection in maintaining species boundaries. Together, our results provide robust genomic support for the revised taxonomy of Orinus and offer new insights into how Quaternary environmental dynamics have shaped speciation and genomic divergence in a montane grass lineage.