The Helicobacter pylori ArsS-ArsR two-component signal transduction system, comprised of a sensor histidine kinase (ArsS) and a response regulator (ArsR), allows the bacteria to regulate gene expression in response to acidic pH. We expressed and purified the full-length ArsR protein and the DNA-binding domain of ArsR (ArsR-DBD), and we analyzed the tertiary structure of the ArsR-DBD using solution nuclear magnetic resonance (NMR) methods. Both the full-length ArsR and the ArsR-DBD behaved as monomers in size exclusion chromatography experiments. The structure of ArsR-DBD consists of an N-terminal four-stranded beta-sheet, a helical core, and a C-terminal beta-hairpin. The overall tertiary fold of the ArsR-DBD is most closely related to DBD structures of the OmpR/PhoB subfamily of bacterial response regulators. However, the orientation of the N-terminal beta-sheet with respect to the rest of the DNA-binding domain is substantially different in ArsR compared with the orientation in related response regulators. Molecular modeling of an ArsR-DBD-DNA complex permits identification of protein elements that are predicted to bind target DNA sequences and thereby regulate gene transcription in H. pylori.