The growing invasion of ecosystems by invasive alien plants (IAPs) has substantially affected biodiversity worldwide, compromising provision of ecosystem services. In this study, we present evidence of the impacts of an IAP, Robinia pseudoacacia L., on native plant diversity in montane rangelands of South Africa and its threats to grazing, an ecosystem service. We assessed stand characteristics, understory vegetation composition and rangeland condition similarities in invaded and uninvaded sites. We observed a shift in grass communities after invasion by R. pseudoacacia as invaded communities differed by 96% from uninvaded rangeland. Invaded habitat was dominated by nitrophilous, shade-tolerant alien ruderals that follow the primitive C₃ carbon fixation pathway. Nitrogen fixation and light-demanding properties of R. pseudoacacia are likely to be the main factors driving these changes. As a result, range condition was significantly lower in invaded habitats with smaller and dense trees (180 ± 24.3) (mean ± standard error) when compared with adjacent uninvaded habitat (401 ± 24.3). These preliminary findings support an urgent need for sustainable control of R. pseudoacacia as an effective approach to stop further reduction in grazing capacity and losses in livestock production.