Modification of eukaryotic proteins is a powerful strategy used by pathogenic bacteria to modulate host cells during infection. Previously, we demonstrated that Helicobacter pylori modify an unidentified protein within mammalian cell lysates in a manner consistent with the action of a bacterial ADP-ribosylating toxin. Here, we identified the modified eukaryotic factor as the abundant nuclear factor poly(ADP-ribose) polymerase-1 (PARP-1), which is important in the pathologies of several disease states typically associated with chronic H. pylori infection. However, rather than being ADP-ribosylated by an H. pylori toxin, the intrinsic poly(ADP-ribosyl) polymerase activity of PARP-1 is activated by a heat- and protease-sensitive H. pylori factor, resulting in automodification of PARP-1 with polymers of poly(ADP-ribose) (PAR). Moreover, during infection of gastric epithelial cells, H. pylori induce intracellular PAR-production by a PARP-1-dependent mechanism. Activation of PARP-1 by a pathogenic bacterium represents a previously unrecognized strategy for modulating host cell signaling during infection.