The aim of this study was to probe the potential anti-H. pylori activity of the synthetic antimicrobial peptide pexiganan, which is an analog of the peptide magainin, and its nanoparticles (PNPs) that were prepared in our laboratory. To compare their antibacterial effects in vitro and in vivo, studies of H. pylori growth inhibition, kinetics and resistance assays were undertaken. The gastric mucoadhesive efficiency and H. pylori clearance efficiency of pexiganan and PNPs were evaluated in rats and mice infected with H. pylori. The eradication of H. pylori was determined using urease tests and a microbial culture method. We observed that PNPs adhered to gastric mucosa more effectively owing to a prolonged stay in the stomach, which resulted in a more effective H. pylori clearance. In addition, PNPs had greater anti-H. pylori effect than pexiganan in infected mice. The amount of pexiganan required to eradicate H. pylori was significantly less using PNPs than the corresponding pexiganan suspension. The results confirmed that PNPs improved peptide stability in the stomach and more effectively eradicated H. pylori from mice stomachs than pexiganan.