The effect of an aqueous extract from the leaves of Rhododendron ferrugineum (RF) was investigated for its capacity of inhibiting the adhesion of Porphyromonas gingivalis cells to epithelial buccal KB cells. RF was characterized by HPLC (12.1% taxifolin-3-O-β-l-arabinopyranoside, 1.6% hyperoside, 0.9% isoquercitrin, 1.6% chlorogenic acid and a tannin content of 8.7%). Additionally raw polysaccharides (RPS) were obtained from the leaves of R. ferrugineum by aqueous extraction. RF and RPS interacted in a dose-dependent manner (max. 25% reduction at 1mg/ml each) with the adhesion of P. gingivalis by influencing bacterial outer membrane proteins. On protein level a time- and concentration-dependent inhibition of Arg-gingipain activity by RF was observed, while the Lys-gingipain activity remained unaltered. In addition, RF and RPS inhibited the bacterial hemagglutinin. RF affected the P. gingivalis adhesion also by interacting with KB cells in pre-incubation assays of the eukaryotic host cells, leading to reduced bacterial adhesion of about 75%. Gene expression analysis by RT-PCR indicated significant downregulation for arginine-specific gingipain rgpA by RF, while lysin-specific gingipain kgp and fimbrillinA fimA were strongly upregulated. Moreover, pre-incubation with RF abolished the P. gingivalis induced expression of IL-1β, IL-6, IL-8 and TNFα in KB cells. Results of this study indicate that an aqueous extract from R. ferrugineum combines cytoprotective and antimicrobial effects by both downregulating the expression of pro-inflammatory genes and inhibiting the adhesion of P. gingivalis. Thus RF may be potential candidate for the development of an adjunctive antimicrobial approach in the prevention of periodontal diseases.