The widely invasive grass Rhynchelytrum repens (Willd.) C.E. Hubb. invades globally-imperiled pine rockland forests of South Florida. To examine whether its spread into and persistence in undisturbed areas pose a threat to native species, we investigated whether the presence of R. repens was associated with a decline in the diversity and abundance of native pine rockland understory plants. We showed R. repens density class was strongly associated with a reduction in native species diversity, with high density (20% cover) plots having five fewer species than low density (0.2%) ones. When we separated species by functional groups, we found that graminoids were affected much more than other native species, with R. repens cover explaining 23% of the variation in graminoid diversity. Rhynchelytrum repens is indeed causing harm to pine rockland diversity, and we recommend that land managers take action to: (1) treat infestations before they reach 30% cover, and (2) remove infestations of any density adjacent to natural areas.