Lipopolysaccharide (LPS) preparations from the periodontopathic bacterium Porphyromonas gingivalis (Pg-LPS) are thought to require Toll-like receptor (TLR)2 rather than TLR4, a receptor of Escherichia coli LPS (Ec-LPS), for activation of immune cells. However, we previously reported that P. gingivalis lipid A, an immunostimulatory principal component of LPS, and its synthetic counterpart activate cells through a TLR4-dependent pathway but not via TLR2. In the present study, a lipoprotein from Pg-LPS (Pg-LP) was shown to be a principal component for TLR2-mediated cell activation. Pg-LP was separated by hydrophobic interaction chromatography followed by preparative electrophoresis and identified by internal peptide sequencing as PG1828, a putative lipoprotein encoded in the P. gingivalis genome. The N-terminal structure was characterized as a triacylated lipopeptide using mass spectrometry. Pg-LP, as well as Ec-LPS, was potent in inducing IL-8 production in human gingival fibroblasts. From our results, we propose that Pg-LP is a powerful inflammatory factor of P. gingivalis.