SurA is a periplasmic chaperone protein that facilitates maturation of integral outer membrane proteins (OMPs). Short peptides that bind SurA have previously been characterized. In this work, an enzyme-linked immunoabsorbent assay-based competition assay is utilized to demonstrate that binding of such peptides, presented by peptide-tagged phage, mimics binding of biological substrates. Two representative unfolded OMPs, OmpF and OmpG, bind SurA and a core structural fragment thereof in competition with peptide-tagged phage, and with the same order-of-magnitude affinity as the peptides. Additionally, unfolded OmpF and OmpG bind SurA more tightly than an unfolded water-soluble protein, while folded proteins have no measurable affinity, demonstrating a specificity of SurA for OMP polypeptides.