The NFkappaB transcription factor is a key component of immune and inflammatory signaling as its activation induces the expression of antimicrobial reagents, chemokines, cytokines, and anti-apoptotic factors. Many pathogens encode effector proteins that target factors regulating NFkappaB activity and can provide novel insights on regulatory mechanisms. Given the link of NFkappaB dysfunction with inflammatory diseases and some cancers, these effectors have therapeutic potential. Here, screening enteropathogenic Escherichia coli proteins for those implicated in suppressing NFkappaB function revealed that eGFP-NleC, unlike eGFP, strongly inhibited basal and TNFalpha-induced NFkappaB reporter activity to prevent secretion of the chemokine, IL-8. Work involving NleC variants, chemical inhibitors, and immunoprecipitation studies support NleC being a zinc metalloprotease that degrades NFkappaB-IkappaBalpha complexes. The findings are consistent with features between residues 33-65 recruiting NFkappaB for proteasomal-independent degradation by a mechanism inhibited by metalloprotease inhibitors or disruption of a consensus zinc metalloprotease motif spanning NleC residues 183-187. This raises the prospect that mammalian cells, or other pathogens, employ a similar mechanism to modulate NFkappaB activity. Moreover, NleC represents a novel tool for validating NFkappaB as a therapeutic target and, indeed, as a possible therapeutic reagent.