Periodontitis is an inflammatory oral disease that affects a large part of the adult population, causing significant costs and suffering. The key pathogen, Porphyromonas gingivalis, secretes gingipains, which are highly destructive proteases and the most important virulence factors in the pathogenesis of the disease. Currently, periodontitis is diagnosed mainly by mechanical manual probing and radiography, often when the disease has already progressed significantly. The possibilities of detecting gingipain activity in gingival fluid could enable early-stage diagnosis and facilitate treatment. Here, we describe a sensitive nanoparticle-based nanoplasmonic biosensor for the detection of the proteolytic activity of gingipains. Gold nanoparticles (AuNPs) were self-assembled as a submonolayer in multiwell plates and further modified with casein or IgG. The proteolytic degradation of the protein coating was tracked by monitoring the shift in the localized surface plasmon resonance (LSPR) peak position. The sensor performance was investigated using model systems with trypsin and purified gingipains (subtypes Kgp and RgpB) and further validated using supernatants from cultures of P. gingivalis. Proteolytic degradation by proteases in buffer results in a concentration- and time-dependent blueshift of the LSPR band of about 1-2 nm when using casein as a substrate. In bacterial supernatants, the degradation of the protein coating resulted in unspecific binding of proteins present in the complex sample matrix to the nanoparticles, which instead triggered a redshift of about 2 nm of the LSPR band. A significant LSPR shift was seen only in samples with gingipain activity. The sensor showed a limit of detection < 0.1 mu g/mL (4.3 nM), which is well below gingipain concentrations detected in severe chronic periodontitis cases (similar to 50 mu g/mL). This work shows the possibility of developing cost-effective nanoparticle-based biosensors for rapid detection of protease activity for chair-side periodontal diagnostics.