The periodontal pathogen, Porphyromonas gingivalis secretes ~30 cargo proteins to the cell surface via the type IX secretion system (T9SS). The C-terminal secretion signal is cleaved by the system’s sortase and supposedly conjugated to anionic lipopolysaccharide (A-LPS). Two types of LPS have been proposed in P. gingivalis, O-LPS and A-LPS. A-LPS was structurally defined as containing a phosphomannan repeat unit, while O-LPS has an O-polysaccharide comprising a tetrasaccharide repeat unit of Gal-Glc-Rha-GalNAc. The genetics of “A-LPS” biosynthesis have been characterised using a monoclonal antibody (MAb-1B5) with 11 genes being found to be specifically required for the biosynthesis of MAb-1B5 reactive LPS. These genes encode proteins which include seven enzymes required for the biosynthesis of the “linking sugar” and four glycosyltransferases. Since none of these 11 genes have been linked to phosphomannan, the association of phosphomannan with cargo modification and MAb-1B5 reactivity has been questioned. In this study, we show that the form of LPS conjugated to the cargo proteins includes O-polysaccharide. Partially purified cargo proteins were deglycosylated with trifluoromethanesulfonic acid (TFMS) for 5 min only and digested with trypsin or proteinase K. Tandem mass spectrometry analyses of the trypsin digested samples showed the C-terminal peptides of cargo proteins connected to repeating units of O-polysaccharide via a putative 4-sugar branch. Analyses of the proteinase-K treated samples demonstrated the presence of up to 12 repeating units of O-polysaccharide. The results show that the form of LPS conjugated to cargo proteins is actually a modified form of O-LPS. The putative 4-sugar branch would explain the four glycosyltranferases specifically required for “A-LPS” biosynthesis.