Porphyromonas gingivalis (P. gingivalis) is a pathogen causing periodontitis. A rapid assay is described for the diagnosis ofperiodontal infections related to P. gingivalis. The method is making use of gingipains, a group of P. gingivalis specific proteasesas a detection biomarker. Magnetic-nanobeads were labeled with gingipain-specific peptide substrates and immobilized on a goldbiosensing platform via gold-thiol linkage. As a result of this, the color of the gold layer turns black. Upon cleavage of theimmobilized substrates by gingipains, the magnetic-nanobeads-peptide fragments were attracted by a magnet so that the goldensurface color becomes visible again. This assay is highly sensitive and specific. It is capable of detecting as little as 49 CFU·mL−1of P. gingivalis within 30 s. Examination of periodontitis patients and healthy control saliva samples showed the potential of the assay. The simplicity and rapidity of the assay makes it an effective point-of-care device.