Anthropogenic nitrogen deposition has been shown to reduce plant diversity in a variety of ecosystems worldwide. However, the impacts of nitrogen deposition on the diversity of many plant communities, such as those of Mediterranean-type ecosystems, remains relatively unexplored. California’s coastal sage scrub, a Mediterranean-type shrubland, is one of the most threatened plant communities in the United States and a biodiversity hotspot. Our previous work, and that of others, has demonstrated that elevated nitrogen deposition negatively impacts coastal sage scrub in a number of ways, such as promoting nonnative plant invasion and exacerbating drought-induced shrub mortality. However, despite a rich body of literature documenting nitrogen-induced changes to this plant community, impacts on plant diversity are poorly quantified. We evaluated patterns of plant richness of coastal sage scrub across the Santa Monica Mountains National Recreation Area of southern California, the largest urban national park in the country. While this park was created in part to serve as an “air shed” of protected land in the region, it experiences a strong gradient of atmospheric nitrogen deposition due to its proximity to urban Los Angeles. We measured soil nitrogen, plant cover, and plant richness at 30 sites of mature, intact coastal sage scrub across this gradient and used regression analyses to explore relationships between vegetation parameters, nitrogen deposition, and other environmental variables. Soil nitrogen availability was positively correlated with rates of deposition. We documented significant declines in native plant richness with increasing nitrogen deposition across the mountain range, primarily due to decreases in native forb species. Increased nonnative plant cover at high deposition sites and aridity also appeared to contribute to reduced native richness. Overall, these results highlight the severe threat posed by anthropogenic nitrogen deposition to the conservation of biodiversity in this already imperiled ecosystem, as well as other Mediterranean-type ecosystem worldwide.