The establishment of terrestrial protected areas (PAs) is an effective strategy for conserving plant species diversity across multiple spatial scales—from local to global. This effectiveness can be assessed using the concept of plant community completeness, which quantifies the degree to which the local biodiversity potential (i.e., the site-specific species pool) is realized. Although community completeness offers valuable insights for nature conservation, few studies have applied this metric to evaluate PAs. We present the first global assessment of plant community completeness for PAs. We analyzed 41,980 plots located inside PAs and 28,599 plots outside PAs, comparing species richness, species pool, and community completeness across various ecoregions and biomes. Using the Getis-Ord Gi* statistic, we identified areas of high community completeness and evaluated the effectiveness of PAs in conserving this metric. Furthermore, we determined ecoregions with the potential to increase community completeness by the expansion of conservation areas outside PAs. Significant variations in species richness, species pool, and community completeness were observed both inside and outside PAs across biomes. At the global scale, the threshold for high community completeness was estimated at 0.208. The lowest 10% quantile values of high community completeness were approximately 0.1 for different biomes in the PA assessment. Biome-specific thresholds revealed that PAs effectively conserve community completeness in 57.2% of the study ecoregions but exhibit limited effectiveness in maintaining community completeness in 71 ecoregions. Community completeness outside PAs showed considerable potential for further enhancement. Given the nearly fixed nature of the species pool and its relationship with community completeness, the high completeness observed outside PAs underscores the need to expand conservation areas in 40.7% of the study ecoregions, spanning tropical, temperate, and desert biomes. Additionally, 105 ecoregions show potential for improving community completeness through restoration efforts outside existing PAs. Synthesis and applications: Community completeness is a powerful tool for evaluating the effectiveness of PAs in conserving plant diversity. It is critical to test whether PAs are in high completeness areas, i.e., in appropriate sites, and suggest which areas are in theory better suited for restoration and transformation into PAs.