Platinum nanoparticles (PtNPs) with potent antibacterial activity were synthesized using pectin and sodium borohydride as capping and reducing agents, respectively. For the first time, the in vivo antibacterial activity of PtNPs was demonstrated using adult zebrafish as the animal model. As a proof of concept, zebrafish infected with a model pathogen, Escherichia coli and a fish-specific pathogen, Aeromonas hydrophila, were subjected to treatment with PtNPs. A bacteria colony count assay revealed that the PtNPs exhibit dose-dependent inhibition of bacterial proliferation and rescued zebrafish completely from bacteria infection. The mechanism of antibacterial action includes the loss of membrane integrity and generation of reactive oxygen species. Toxicology studies reveal that the antibacterial concentration of PtNPs used in this study is non-toxic to zebrafish. Being non-toxic to zebrafish, these PtNPs might open up new avenues in antimicrobial therapy for future biomedical applications.