Many Prosopis species have been introduced to South Africa; some taxa and their hybrids have naturalised and become widespread invasive trees. These invasions have detrimental effects on biodiversity, ecosystem services and human livelihoods. Although several studies have documented these impacts, the studies have been limited to single sites or restricted areas. This study assessed the Prosopis population across the full invasive range of the genus in South Africa, and quantified the effects of invasions on native woody and herbaceous species. Basal areas of invasive Prosopis stands reached 9 m2/ha, and were on average higher along perennial rivers than along ephemeral rivers (mean basal areas of 3.2 vs. 1.4 m2/ha). Native woody species density, basal area, richness and diversity all decreased significantly as the basal area of Prosopis stands increased. For example, up to eight native woody species occurred at basal area of < 2 m2/ha, this decreased to three native species or fewer at basal areas of > 4 m2/ha. The cover of native perennial grasses and herbaceous plants declined from 15–20% where the basal area of Prosopis was < 2 m2/ha to zero where the basal area of Prosopis was > 4.5 m2/ha. The results highlight the widespread nature of the impacts across all invaded biomes. Current control of Prosopis has had limited success, and alternative, potentially more effective, options are controversial. In the light of the widespread impacts, we recommend that a thorough assessment of the problem be undertaken to inform policy.