Gram-negative bacteria possess an outer membrane (OM) that acts as a barrier against toxic compounds. Lipopolysaccharide (LPS) in the outer leaflet of the OM is crucial for barrier function. After its synthesis in the cytoplasm and translocation to the periplasm, LPS is transported to the OM by the LPS transport system. The LPS translocon, composed of an OM protein LptD and a lipoprotein LptE, mediates LPS assembly into the OM. Recently, the small lipoprotein LptM (YifL) was identified as a novel LptD/E-interactor that facilitates LptD maturation. However, its mechanism remains unclear. Here, we investigated the detailed interaction between LptM and LptD. We found that LptM interacts with the folded LptD intermediate at the late stage of its maturation. Mutational analyses demonstrated that the N-terminal conserved region (C 20 GLKGPLYF 28 ) of LptM is essential for its function. Cryo-EM structural analysis of the E. coli LptD/E/M complex, combined with biochemical analyses, revealed the molecular basis of the LptM–LptD interaction and its functional importance. Thus, we propose that LptM functions as a “barrel-rivet,” stabilizing LptD for its proper assembly.