The bacterial type II secretion system (T2SS) is unique in its ability to promote the transport of large folded and sometimes multimeric proteins. In this secretion process, exoproteins are first translocated into the periplasm. The final release into the medium requires a multiprotein complex called the secreton. Although the 12 individual components of the secreton have been identified, its mode of action remains obscure. We set up various dedicated in vitro and in vivo protein-protein interaction experiments to identify the Pseudomonas aeruginosa Xcp T2SS periplasmic interactome. BIAcore experiments revealed that three Xcp components, XcpP, the secretin XcpQ, and the pseudopilus tip, directly and specifically interact with secreted exoproteins. Affinity chromatography co-purification indicated that the XcpY periplasmic domain interacts with the secreted substrate and a component of the pseudopilus tip XcpW. Interestingly, the periplasmic domain of another member of the Xcp inner membrane platform, XcpZ co-elutes with the XcpY/substrate and the XcpY/ XcpW complexes during affinity chromatography. Finally the direct interaction between the secreted substrate and XcpY was confirmed by in situ photo-crosslinking. All together, our results allowed us to propose the most advanced integrative model of Xcp T2SS assembly and function.