The Type IX secretion system (T9SS) is specific to Bacteroidetes and is required in the translocation of virulent proteins that possess a conserved C-terminal domain (CTD), across the outer membrane (OM). On the surface the CTD signal of these proteins is cleaved prior to modification with anionic lipopolysaccharide (A-LPS). This enables the attachment of the 32 CTD-containing proteins essential for virulence to the cell surface of Porphyromonas gingivalis, a periodontal pathogen. At least 14 proteins have been identified as components of the T9SS including PorK, PorN and PorU, however the structural organization of these proteins has not been elucidated and the exact mechanism of attachment of the secreted proteins to the cell surface is unclear. In this study, using cryo-tomographic electron microscopy we show that PorK and PorN form a 50 nm diameter ring-shaped structure containing ~32-36 subunits of each protein. Cross-linking and mass spectrometry (MS) analyses of the purified complex revealed that PorK/N interact with the PG0189 OM protein which we show to be a novel component of the T9SS. These results suggest that the ring-shaped PorK/N complex may form part of the secretion channel of the T9SS. Additionally, we show in the absence of A-LPS that secreted CTD proteins were released into the culture fluid with their CTD signal cleaved. These CTD proteins were not modified with A-LPS however, MS analysis revealed the presence of various peptide modifications at the C-terminus. Together, this suggests that the CTD signal is cleaved and the CTD protein forms an intermediate bond with a transpeptidase (PorU) which is resolved by a free amine from A-LPS, thereby attaching CTD proteins to the cell surface, a mechanism similar to sortase A. This is the first report showing (i) the structural organization of any T9SS component and (ii) a sortase-like mechanism in the Gram-negative bacteria.