Titanium and stainless steel substrates were modified by grafting with mercaptododecylphosphonic acid (MDPA) followed by reaction with silver nitrate (AgNO₃), in order to investigate the potential of phosphonate self-assembled monolayers functionalized by silver thiolate species as antibacterial nanocoatings for inorganic biomaterials. The samples were characterized by Fourier transform infrared (FTIR) spectroscopy in grazing-incidence mode, water contact angle measurements, and X-ray photoelectron spectroscopy (XPS). The influence of the surface modification on bacterial adhesion and biofilm growth was investigated in vitro using Escherichia coli, Pseudomonas aeruginosa, Staphylococcus epidermidis, and Staphylococcus aureus strains. The stability of the monolayer in blood-mimicking medium was examined. Despite their very low silver content, MDPA + AgNO₃ monolayers strongly decreased bacterial adhesion (>99.9% reduction in the number of viable adherent bacteria) and biofilm formation in comparison to the bare substrates.