AbstractPremiseAnthropogenic activities such as fossil fuel combustion and synthetic fertilizer synthesis have resulted in nitrogen (N) deposition and elevated N availability in ecosystems. Many parasitic plants are adapted to low N environments and have evolved mechanisms to sequester water, N, and other limiting nutrients from hosts. Anthropogenic N deposition may perturb these host–parasite interactions, thereby altering nutrient cycling and ultimately reducing biodiversity.MethodsTo test how N enrichment affects the incidence and degree of plant parasitism, we assayed host–parasite performance and in vitro root growth under supplemental N levels representative of urban areas experiencing N deposition. We focused on the annual hemiparasite Castilleja exserta and two co‐occurring host species, Nasella lepida and N. pulchra.ResultsElevated N decreased haustoria formation and host‐seeking behavior by roots but did not affect growth, suggesting N enrichment may reduce parasitism without associated loss of parasite biomass. We confirmed that parasitism decreased host performance and that the degree of parasitism was positively related to host biomass and earlier flowering. We also found N. lepida may be preferred over N. pulchra as a host.ConclusionsThese results indicate that N pollution altered parasitism in C. exserta and highlight the far‐reaching ecological effects of N pollution on host–parasite interactions within plant communities.