Soy protein has shown potential as a renewable and environmentally friendly adhesive because of its superior performance and affordability compared with urea formaldehyde-based adhesives, but poor water resistance has limited its application as a high-performance wood adhesive comparable to phenol formaldehyde. This work focused on developing and characterizing undecylenic acid (UA)-modified soy proteins to improve their water resistance. The reaction between amine groups from protein and car-boxyl groups from UA was proposed to be the main chemical pathway for grafting, which was proven by Fourier transform infrared spectroscopy (FTIR) and a ninhydrin test. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) showed that UA modification led to reduced thermal stabil-ity because of protein unfolding and decreased protein-protein cross-linkages. The increased attractive force between carbon chains of UA and protein residues resulted in higher viscosity and dynamic mod-ulus values. Atomic force microscopy (AFM) images showed changes in protein particle size and surface properties. The wet strengths of modified soy protein adhesives were significantly improved by 35–62% compared with the control. UA with hydrophobic carbon chains and reactive carboxyl groups is an ideal bio-based modifier for soy proteins.