A series of crosslinked waterborne polyurethane (WBPU) adhesives were prepared by prepolymer synthesis. Modified melamine, hexamethoxymethyl melamine (HMMM) was used as the crosslinking agent. It was elucidated by FTIR and 1H-NMR spectroscopy that the crosslinking reaction occurred between the polyurethane carboxyl acid salt groups and the HMMM methoxy groups. The hydrophobicity of the WBPU films increased after HMMM crosslinking. As the HMMM content was increased (increasing mole ratio of HMMM/dimethylol propionic acid (DMPA)), the water uptake (%) of the film decreased, and the water contact angle increased. The thermal stability, tensile strength, and initial modulus increased with increasing HMMM content up to an optimum value (mole ratio of HMMM/DMPA=0.5) at which point the maximum thermal stability, tensile strength, and initial modulus were recorded. The adhesive strength was found to be dependent on HMMM content, curing time, and temperature. The adhesive strength of crosslinked WBPU in the case of optimum HMMM content (8.46 wt%) was only slightly affected after immersing adhesive bonded nylon fabrics in water (for up to 2 days).