The gastric human pathogen Helicobacter pylori faces formidable challenges in the stomach including reactive oxygen and nitrogen intermediates. Here we demonstrate that arginase activity, which inhibits host nitric oxide production, is post-translationally stimulated by H. pylori thioredoxin (Trx) 1 but not the homologous Trx2. Trx1 has chaperone activity that renatures urea- or heat-denatured arginase back to the catalytically active state. Most reactive oxygen and nitrogen intermediates inhibit arginase activity; this damage is reversed by Trx1, but not Trx2. Trx1 and arginase equip H. pylori with a "renox guardian" to overcome abundant nitrosative and oxidative stresses encountered during the persistence of the bacterium in the hostile gastric environment.