The type IX secretion system (T9SS) is responsible for exporting proteins across the outer membrane (OM) of bacteria in the Bacteroidetes phylum and is essential for the pathogenicity of oral pathogens associated with severe periodontal disease. The PorKN rings are essential for the function of the T9SS, but their precise role and the details of their assembly remain unclear. Here, we present a ~3.5Å cryo-electron microscopy structure of the PorKN rings which revealed that PorK multimers form a rim on the outer edge with a large, flat surface oriented towards the outer membrane, while PorN fold to create cog-like projections on the periplasmic side providing functional insights into the rotation of the T9SS. The PorKN rings are known to associate with the PorG OM β-barrel, however, this component was not resolved in the cryo-EM structure. Using biochemical analysis, mass spectrometry and structural modelling we show that the C-terminal extension of PorG lies within a shallow groove on the OM face of PorK and is stabilized by a disulfide bond. This disulfide bond was found to be essential for the function of the T9SS but not for the formation of the PorKN rings. In the absence of PorG, the PorKN rings were unstable, and the interaction between PorK and PorN was affected. Based on these results, we propose that PorKN rings may have a rotational movement and PorG has structural as well as functional role in the T9SS.